A Tesco near me (one of something like 7 in my town...) recently had massive renovations done to make it far bigger like a true superstore with all the clothes and other outlets that Tesco have now, and so of course there are new staff. Plenty that I saw down there were of student age who I hadn't seen there before (although of course I won't claim to know the full staff quota) so I wouldn't say that they're avoiding such people. It's far more likely that as has been suggested they are simply attempting to employ more people in full time positions at the moment in your area. Keep trying, and go for other places too of course, and eventually you'll find a placement.
Temp agencies are a good place to try too if you haven't already been, they should help you find a job that suits your hours and such.