There is a strong presumption in favour of the singular where interrogatives are concerned, so unless the questioner was sure that multiple people were performing the beating, the form "who was" would probably be used. Even if such certainty existed, it is possible that the questioner would still say "who was".
The Cambridge Grammar of the English Language says that "in general, the interrogative pronouns who and what take the default value of singular".
One of the example is "Who wants some more ice-cream?", and as the Grammar points out, there is no presumption that only a single person wants it. It is easy to imagine that the mother or father of a large family might feel certain that more than one individual wants ice-cream, but the question will still usually be "Who wants" and not "Who want".
The "default singular" can be "overridden" though if the speaker presupposes a plural answer (an example given is "Who haven't handed in their assignments?"). This override is optional, unless a plural noun later in the sentence forces the override: "What are going to be the deciding factors?".
Thus we can distinguish three situations:
1) "Who was beating the boy for not following rules?" - This is the singular default that we use if we don't know how many people were responsible.
2) "Who was/were beating the boy for not following rules?" - If we know or strongly suspect that there was more than one culprit, there is an optional plural form.
3) "Who were the kids beating the boy for not following rules?" - Plural override is compulsory because the plural noun "kids" expresses the plurality of the culprits.