My teeth are yellow. So I'm planning to brush my teeth daily onlce at night but after school I brush it 1 hour in a row. Is there anything bad about it if I do it in a row. (1 hour)?

Are you saying you are going to brush your teeth for a solid hour after school? If so, it will not make any difference and you may do more damage.  Think about all that brushing against your gums. 

First you need to brush more than just at night.  I suggest you brush them in the morning too. You should know that just brushing will not make them white again. But you be nicer to be around.

Incorrect or excessive brushing can actually wear down the enamel on your teeth.

I assume your teeth are stained.  That may not be reversed by just brushing.

Ask your dental hygienist how often and how long to brush