If this is for school, then I suggest you a) use a search engine to find "average weight of Buff Orpington cockerels", b) locate a Buff Orpington chicken society, or c) locate Buff Orpington breeders.
Breeders and people who keep up the breed standard keep careful records - you might have to inquire directly if they have not posted such information online.
Make sure that you are judging your cockerel by the same breeding standards of its peers in your country, although such information can be included for your study. (UK as opposed to USA.)
Proper research for a study requires that you use sources that can be verified. Real names, of real authorities on the subject. Not screen names.