Prosecutors say Sean Corragio, 22, engaged in a sexual act in a mall with a 14-year-old he met online.
Corragio was arrested after police executed a search warrant at his Shirley home and according to prosecutors found a "significant amount" of child pornography on multiple computers and cell phones.
Corragio was previously employed as a school security guard in the Longwood and William Floyd School Districts. The charges are unrelated to his work at the schools, officials said.
His lawyer said Coraggio is a Marine reservist with no priors. He was held without bail and has been placed on suicide watch.