Which site would be the most reliable source for an essay about bullying in schools today?

Always cite sources that use .org .edu or .gov 

try to avoid sources that have excessive amounts of adds, pop-ups, or end in .com or .net

try to cite from peer-reviewed scientific journals if possible

make sure the information is not outdated

always make sure that the source you are using also cities where it got its information, and check those for validity.

For this particular prompt, it is a good idea to look at studies and government information.