Certificate for filtering system

We have recently moved for LLN which was providing us with access to internet and doing all the filtering, we now do all thios ourselfs. The rpoblem im having as that to get on the internet a certificate needs to be installed in the keychanin otherwise students are unable to access a lot of features on the internet and also unable to access the app store. I was wondering if anyone could tell me a way that i could install this certificate into the keychain either when a user logs on or all at once.