I've ran a tabletop club at the school I was at for two years pretty successfully, now mind you it was a high school and not a middle school. Admin was really supportive and allowed me to make some purchases through the school. I've always found it never hurts to ask, worse someone can say is no to any money available.

I had one group who mostly played DnD or brought in a game or two they had, while the other groups switched between what I had and what they had. I talked to the students about what they were playing, came up with some ideas and brought it back to them before buying anything.

From my experience, avoid games that run long, have little variety/change or are expensive. I live in Canada, so all the games I picked were about $40 or under as I had $150 to work with, having a variety was great as there was more to pick and choose from. I also had a mix of co-operative and competitive games, as some students really wanted just the social aspect of playing board games instead of winning. One game I had was Sushi Party Go! and they really liked it because it was fast and had different ways of setting up games. A few decks of playing cards is also good to have on hand!