怎么表达"提前交卷"

xbadx

怎么说:" 你提前交卷，其他人还以为你是不会做呢，
因为那么多题，大家都觉得不可能提前半小时写完"

it is something about accounting exam, some one submit his exam paper______
(prior to the finish time?); because, there are so many questions, most of us think none should be able to finish it _____________(half an hour early)?

关键是“提前半个小时“不会表达

Someone handed in the exam paper half an hour before the given time was up. As there were too many questions, most of us thought that no one should be able to finsih it earlier than the time given.

I think it should be like this. But, still...I think that my sentence looks weird.