I think it's safe, everyone at uni has their own stuff anyway..why would they steal yours? Plus, it's a library. - surely people are just studying and therefore stealing would not be on their minds at all. I personally leave my stuff for agesss in the sixth form library all the time. Like, my bag + I even leave my phone on the desk- go and have lunch then return and nobody's ever touched anything ever. Idk if sixth form libraries are comparable to those at uni but I dont think there's anything wrong with it.