Most student teachers undertake 2 - 3 practical experience sessions throughout their degree, these are usually arranged by the University on your behalf. If, like me, your university does not arrange practical experience and you need to approach schools yourself try treating it like any other job interview. You may need to be firm to avoid principal's from dismissing you, but remain polite and professional. For many principals being approached directly by a student is unusual and they are often reluctant, claiming that they only deal directly with the University. I have found cold calling is better than sending a letter that can be easily ignored and if you have them, use your connections. Friends or relatives that work in schools, teachers of your own children can all help break the ice with principals, this is how I have arranged two of mine thus far.
Good luck.