Is this sentence active or passive. the custodian repaired the classroom door?

Both active and passive sentences have an action, but only active sentences show very specifically which particular person or thing is doing the action. 

This sentence would be passive if it said "The classroom door was repaired," because then we wouldn't know WHO or WHAT repaired the door.

The sentence would also be passive if it said "The classroom door was repaired by the custodian." Such a sentence places unnecessary distance between the agent (the person doing the action) and the action itself. Why say the door was repaired by the custodian when you could simply say the custodian repaired the door?

So the sentence in your question is definitely an active sentence.