Being involved with pupils in a school is a different context to being involved with children in church. You may be the only Christian in contact with the school, so it is vital that you build positive, reliable and trustworthy relationships and act appropriately.
Anyone going into school to work with pupils needs to be checked by the Criminal Records Bureau. If necessary, the school can arrange that for you.
You will find advice here on child protection, approaching the school, handling issues of faith in school and appropriate dress code. Schools will judge the wider Christian church by you as their known representative!