I discovered the 1968 reprint of this Betsy Thoughtless a while ago when I was searching for copies of Haywood's The History of Miss Betsy Thoughtless and immediately purchased a copy. So, you may be wondering—like me—whether this story bears any relation to Haywood's Betsy Thoughtless. In a word, no. Ashbee characterises the novel thus:
Betsy recounts how she lost her maidenhead, and that she applied the rod to her cousin-lover at his own request. [The publisher] designates it in his catalogue as: "a most spicey and piquant Narrative of a Young Girl obliged to excoriate her Sweetheart's bum, before he could ravish her Maidenhead."
None of the modern authorities do any more than quote this snippet from Ashbee, whose entry fills six pages (quoted at length, below). The Erotica Bibliophile (here) provides bibliographical details, but these are also taken from Ashbee, adding only a reference from: Peter Mendes, Clandestine Erotic Fiction in English, 1800-1930: A Bibliographic Study (Aldershot, Hants: Scolar Press, 1993), 118-A. Mendes locates copies of the 1917 edition, which is not uncommon (copies are available online from ca. A$170.00), but I can't find copies of the earlier editions. The 1968 Grove Press reprint that I bought is very widely available for under A$10.00.
As, seemingly, the only person to read this particular Betsy Thoughtless in many decades, I thought I should say something other than "spicey," "piquant" and "excoriate" (excellent as these words are). The story proper is contained in a letter, which has a brief introduction, as follows.
I became acquainted with this lovely girl during my visit to Cheltenham. She is the daughter of a rich commoner in the West of England, and we soon formed an intimacy of the sweetest and most agreeable kind. She was in her eighteenth year, with a form and face seldom equalled: her hair was of a lively brown, her skin perfectly white, and her face full of ardour and beauty. I discovered her secrets very soon after my introduction, by the recital of a warm and very libidinous story, which I read in the French, and which I pretended to have witnessed. This won her confidence, and she confessed that she been seduced by a young gentlemen, a protégé of her father’s.
I lent the beautiful creature some luscious prints, tastefully executed, and which made her almost mad, as she had never seen anything of the kind before. In return for the these, which I afterwards gave her, she promised me, in writing, and memoir of the circumstances and causes of her seduction, and I cannot do better than give it in her own words:
To Miss Wilson.
My Dear friend,
’Tis well thet I am alone, for I blush to the ears in making a confession, even to you, of the wanton and naughty scenes which led to the first indulgence of my passions;—passions the most ardent and glowing; but which, I am happy to say, have met with a corresponding feeling in those of my lover;—but to my tale.
This letter is signed and dated:
I remain, my dear friend,
Yours very faithfully,
Since this post is already getting rather long, the contents of Elizabeth Thoughtless's letter to Miss Wilson will have to wait for another time, along with a discussion of the 1788 edition of Venus School Mistress. But before I finish, here are the relevant bits out of Ashbee's entry on Venus School Mistress.
* * * * *
Ashbee, Index Librorum Prohibitorum (1877), 397–99, 401:
Venus School Mistress; or, Birchen Sports. By R. Birch, Translator of Manon's Memoirs. Printed for Philosemus. Embellished with a Beautiful Print. Price 10s. 6d. 8vo.; pp. 82; probable date 1808 to 1810; this I take to be the earliest edition; it has an "Address" in which reference is made to the success of "Manon la Fouëtteuse," and contains the history of "Betty Thoughtless." There is another edition about 1820, "with 4 coloured plates, 8vo. 16/-." [fn. 1]
About 1830 Cannon reprinted the work, with title "Venus School Mistress; or Birchen Sports. Reprinted from the Edition of 1788,[fn.2] with a Preface, by Mary Wilson, containing some Account of the late Mrs. Berkley. London: Printed by John Ludbury, No. 256, High Holborn.," and a second title page thus: Aphrodite Flagellatrix: Sive Ludi Betulani. De gustibus non est disputandum. Romæ: Apud Plagosum Orbilium, In Viam Flagrorum, Sub Signo Flagelli. 1790." Large 12mo.; pp. xi and 58; on the first title page is a hand brandishing a rod, and on the second a Roman lamp; 5 or 6 folding coloured plates, pretty well done, and a frontispiece (not folding) representing "The Berkley Horse." [fn. 3]
W. Dugdale issued an edition about 1860, with titles as above, except that on the English title page the printer's name is replaced by "Printed for the Booksellers," and there is a fleuron of the Royal Arms. 8vo. ; pp. 61 ; 8 coloured plates, badly done, and not copied from those of Cannon's edition;
Price Two Guineas. In both these editions the original "Address" is omitted, and a "Preface," as mentioned on their title pages, added; both contain "Betty Thoughtless."
"Venus School Mistress," a worthless, badly written book, contains the experiences of "Miss R. Birch," daughter of a woman who kept a day school, and who never let pass an opportunity to flog her pupils. Miss Birch acquires a great taste for, and aptitude in laying on the rod, and eventually takes a school herself together with a friend. "We now (she concludes) live together, and whip like two little devils, both young folks and old ones." The adventures all turn on flagellation and are generally dull.
The tale of "Betsy Thoughtless" is insignificant. Betsy recounts how she lost her maidenhead, and that she applied the rod to her cousin-lover at his own request. Dugdale designates it in his catalogue as: "a most spicey and piquant Narrative of a Young Girl obliged to excoriate her Sweetheart's bum, before he could ravish her Maidenhead."
In the Preface to Cannon's edition the topic is treated seriously. We are there told: "The subject of Flagellation, in venereal affairs […]" The remainder of the preface, as notified on the title page, contains an account of Mrs. Berkley and her establishment, which I have already quoted in my introduction.
 Publisher's Catalogue of the time.
 This date appears to be fictitious. "Venus School Mistress " was issued after "Manon la fouëtteuse" to which reference is made in the "Address," (vide supra), and "Manon" could not have been originally published before 1805.
 Reproduced at p. xliv, ante.
[UPDATE: 2 July 2016: After all my pictures disappeared (again) I decided to give up on external hosts for large versions (1000px) of my image files and, for now on, will stick with the smaller images (500px), which Blogger is prepared to host.]