Hardly had the bell nmg--- I reached the class.

The correct word to complete the sentence is "when." The sentence structure here is using an inverted order with "hardly...had" to indicate that the action of the bell ringing is completed just before the action in the main clause (reaching the class). So, it should be "Hardly had the bell rung when I reached the class." This means that the action of reaching the class immediately followed the action of the bell ringing. Therefore, "when" is the appropriate word to show the sequence of events in this context.