"The students have to explain their answers."
Μετάφραση:Οι μαθητές πρέπει να εξηγήσουν τις απαντήσεις τους.
There is no need of agreement as the subject of an impersonal verb (πρέπει, πρόκειται, γίνεται, είθισται and more) is a full sentence. That's why it's impersonal! :) These verbs always appear in 3rd person singular. In the example "να εξηγήσουν...", i.e. the whole secondary clause, is the subject of πρέπει. The only agreement needed is between οι μαθητές and να εξηγήσουν as subject+verb. So you could rearrange it as "Πρέπει οι μαθητές να εξηγήσουν τις απαντήσεις τους." which is fine, but in sentences like the one above usually you see that split: the subject of the secondary clause goes before the impersonal verb and away from the subjunctive.