Oh, a prank war. I miss the prank wars we had back in school
Well anyway, for that alarm idea. It's best to do it when he is going into a private room. Such as his bedroom (where he does not expect anyone else to be).
But since this war might be bound to only school property?
We will need other methods...
Now pulling a prank at school can be tricky. Because you don't necessarily want yourself to get in trouble.. Maybe your friend can get into a little heat but not you. Getting a "referral" or a "write-up" is no big xD
One that I like, but involves getting your victims address. Is sending him some... Gross, products.
PoopSenders
It is just as it sounds.. Best part is you can send a good amount for just 12 dollars and shipping. Which for a prank that leaves the person totally clueless as to why someone or who sent him a box of crap... Is just awesome.
You could put an "annoy-a-tron" from Thinkgeek.com
Link
The are pretty sweet, let out random beeps at random intervals.
Having this happen enough will drive you mad haha.. I would get a few of them just in case.
You can also take a song he might like but has not heard (maybe the song is new). Open it in a program called Audacity
Link and about 15 seconds in. Take a screaming female (high pitch) and replace the rest of the song with it.
Save this in an audio file your mp3 player can play.
(I recommend cranking up the scream up a couple dozen dbs(volume))
Then tell your friend to listen to the song, that it is a "really cool song".
He will probably pee his pants (my friend almost did xD)
You can pull off the mentos in coke bottle trick.
Youtube
EDIT:
To be honest, I've seen them at the Dollar store...yes, a single dollar
However, there's some that's around 5-15. The $15 ones have three of them...so if you want carnage...you know what to get
And seven cents!