Can I use one essay for multiple schools?

It depends on how similar the colleges and the prompts are. Be aware that similar-sounding questions often have subtle variations. Make sure you're answering each school's prompt directly and completely. You also may want to consider differences in the type of students the colleges are seeking, and what your strategy is for each school. Of course, if you do decide to send the same statement to multiple schools, be sure to proofread carefully to make sure you don't mention X College in your statement for University of Y.