Seems right to me. You don't want high level monsters to get disabled easily. On the other hand, because of bound accuracy, lower level mobs are still quite dangerous. Those are the types of creatures spell casters should be wiping out first (in a mixed level battle).
In the PHB, there are rules for downtime actions. One use of downtime is to learn a skill. So, if you are not proficient in thieves tools, you could spend some gold and time to become proficient and thereby raise your skill without killing anything.
Other Ideas
1. Student steals a dark lore book and accidentally opens a portal to another dimension releasing creatures and sucking some students/instructors to the other side.
2. A cockatrice is loose! Adventurers have to follow the trail of statuary to recapture it.