Is this a right sentance???? PLEASE ANSWER?

Question

After a couple of warnings students are still acting childish

like when teachers notice them using their phones they start laughing even though they are old enough to know THAT THAT is wrong.

Answer 1

First of all, learn how to spell.  It's sentence, not sentance.  And, no, it's not.  It should read something like this:

Students are still acting childish, even after several warnings.  For example, they are using their cell phones and being disruptive in class by laughing out loud. They are old enough to know better.

Answer 2

"After a couple of warnings, students are still acting childish. Just like when teachers notice them using their phones, they start laughing even though they are old enough to know that it is wrong."

I think that this is the best i can make out of your sentence.

Answer 3

No I'm sorry but you aren't even close.  You are rambling.  Don't capitalize any words but the first one in the sentence.  Remove the word like.  If you have your English book you need to study a bit more.  

Answer 4

"The students are still acting childishly after a couple warnings. They start laughing when teachers notice them using phones even though they are old enough to know this is wrong."