Totally Spies! (2002)

S7E16 Something's Fishy

The Spies are taken aback to discover that Zerlina is the new teacher at AIYA. However, she's on a secret mission to investigate a shadowy individual who's been stealing from the campus. This puts the Spies in a position to join their boss on a stakeout, adding pressure as they know they must not fail in their task.