Lost. As you’re using the past tense. You already found the bag, therefore somebody will already have lost it

Better in some ways is If you’ve lost a bag … This is because you’re not asking a general question about losing bags in the past, you’re asking about the present (not having a bag now) To use the present tense, you can say If you are missing a bag…

Both "lost" and "lose" can be used in this context, but they have slightly different meanings: "If you lose a bag, please ask the teacher there for it." This suggests a general situation where someone might lose a bag in the future, and you're giving instructions on what to do in that case. "If you lost a bag, please ask the teacher there for it." This suggests that someone has already lost their bag, and you're instructing them on what to do now. So, which one you choose depends on the context. If you're giving a general instruction, use "lose." If you're referring to a specific incident or person who has already lost a bag, use "lost."