I suspect this has more to do with the gross, salacious way stories of female teachers who sexually abuse their students are treated in the media than the actual numbers.
It was surprisingly difficult to find real statistics on this, but I did turn up this 2004 report prepared for the US Department of Education, Educator Sexual Misconduct: A Synthesis of Existing Literature. The section on sex of offenders begins on p. 24 of the document, p. 32 of the PDF. Seven studies on this subject are discussed, and while the results varied quite a bit all found more male than female offenders. (See Table 8 in the report.) Studies that looked at public records found far more male offenders (from 80% to 96%). In two studies that involved questioning students themselves, the numbers were closer. Both studies found that 57% of offenders were male and 43% female.
FWIW these numbers do include school employees other than teachers, such as principals, counselors, and bus drivers. The previous section of the report indicates that most offenders are teachers and coaches, but I don’t see that it has a breakdown of teacher offenders by sex.