Mobiles can be a great asset but they can also cause problems at school.  The following information comes from the Ofcom (UK) website -

Mobile parental controls

• All mobile phone providers offer free parental control services which limit the content children can access via the mobile network to items suitable for under 18s. However, they may not always be automatically switched on. Check with your service provider that the parental control settings are switched on, and ask for them to be switched on if they are not. This is particularly important if the phone was used by an adult before.