Yes, you have a First Amendment right to the free exercise of religion. What this means is that the government cannot stop you from practicing your religion, which includes saying a prayer. 

However, the First Amendment also prevents the government from forcing you to follow a particular religion or to be religious at all. This is called the establishment clause. Because of this, a public school cannot say that one religion is better than another or even say that religion is better than no religion. A public school also cannot force any student to pray.