Hi there! I'm tryi'n to create rollup trigger by aggregate sum of fields

i have 2 custom objects with lookup field School__c on Classroom__c(child) ,objects:

-School__c with fields,Number_of_students__c,Name

-Classroom__c with fields,Students_Quantity__c,Name and lookup field School__c

How can i have a trigger(before insert,after update,before delete)?

thanks for all advices!