i think its not bad as u have admitted urself ur brother is not that bright!
it is said : *latoon kay bhoot batoon say nahi mantay! *
so if ur brother fall in that category i think teacher is justified. but if he is serious in studies but have some serious learning difficulties so u shud help him.
beside such kind of mild punishment does not adversely effects the boys rather they make them bit ruf and tuf........ and i think this is gud and make them more confident.