Looking at it from the many years of experience as a convenience store manager:
Shoplifting is pretty much right out. Theft correlates more strongly with socioeconomic status than age, and while students have a low socioeconomic status per se, they also share their parent’s status to a large degree.
I wouldn’t even look very hard for shoplifting, because it’s not very efficient - less than ten percent of inventory shortage, and virtually zero cash shortage, is caused by the customer. The greatest source of inventory shortage, and almost the sum total of cash shortage, is your own employees (roughly half simple counting error, roughly half intentional theft)
There are a couple of better reasons, namely the aforementioned large groups and obnoxiousness. Efficiency drops with high customer count, and let’s face it, while most teens are decent and reasonable people, more than a few are obnoxious little bastards who will drive off other customers.
But limiting the number of students is still the wrong way to do handle the situation. Even better, the store should have more staff on hand for customer rushes (in this case, lunch and after school) - all registers open with good fast cashiers, one or two people stocking (really, keeping an eye on people), and the manager present to handle any problems as quickly as possible. Limiting their numbers accomplishes nothing but losing their custom, both in the immediate sense (they would buy something if allowed to) and in the long run (if you deny their custom as teenagers, you’ll never recapture it later)