Here are some mild jokes: (As u mentioned it is for 'school' magazine :P )
1) What do u call a fish without an eye?
2) What goes tick-tock, bow-wow, tick-tock?
Ans: A watch dog.
And some riddles:
1) What word becomes shorter when adding two extra letters to it?
2) What word starts with 't', is filled with 't' and ends with 't'?
3) We see it once in a year, twice in a week and never in a day. What is it?
Ans: The letter 'E'.
I hope I helped. :)