Is us juniors the correct way to complete a sentence?

It can be. If this phrase is at the end of a sentence, it is probably the object of a preposition: The seniors had enough money to go on a field trip, but there was not enough money left for us juniors. ("For us juniors" is a prepositional phrase.) If you wanted the juniors to be the subject of the sentence, it would look like this: We juniors are very disappointed that there wasn't enough money for us to go on the field trip.