Dresses would work, too.
This used to happen to me at that age, but it really wasn't a sexual thing; it was more a buggin' for fun thing. If they couldn't go for the waistband they would go for the neck (we wore a lot of back-zipped dresses in the 60's.) I dealt with it by simply slamming back into my seat as hard as I could when the hand got into my space. Naturally, little Johnny would screech, and when the teacher would come over, I was perfectly within my rights to have leaned back in my chair--he was the one whose hands were somewhere that they shouldn't be. Once I managed to break a boy's finger; he kept his hands to himself after enduring the ER visit that ensued. (To say nothing of his father's wrath over how much it cost.)
If the kid is reaching through the desk rather than over it, ask the teacher to allow you to tie a pillowcase over the seatback, and wire down the corners so that it stays down. That way his hands will have to be on top of the desk to get anywhere, and there is more chance he'll get caught.
PS: Notice I didn't talk about defenses that involve hitting back using hands; that would undoubtedly get a kid in trouble. When I was little the rule was that they didn't care who swung first; anyone who struck out at anyone got in trouble. I was the kind of kid who worked out all the ramifications before I used a defense, and that particular one worked, because I kept my hands and feet to myself and never reached over the edge of my desk.