A passive form of "people were always laughing at the little girl" makes sense: "the little girl was always being laughed at" (on se moquait/riait toujours de la petite fille)
The passive form of your sentence would be "the little girl is always being laughed at by the boys", and it makes sense too if "the little girl" is what/whom you'r talking of. (la petite fille était la risée des garçons)
I don't know why on earth one would want to say a thing like that in the passive, unless it's part of a school exercise, so I'll give you a method rather than a straightforward answer.
1 - In your original sentence, what is the object, what is the subject?
Object: the little girl; that will become the subject of your passive sentence (S)
Subject: Those boys; that, if necessary, will become the agent: by those boys (A). Note that very often, the passive is used in English to avoid naming the agent.
2 - Observe the tense: are laughing; this is the present continuous. So you will use the present continuous of the passive auxiliary, be - you have a choice of am / are / is being, depending on the subject. This will be (AUX). Make your choice now.
3 - The past participle of the verb in the original sentence is now necessary: to laugh at. You're lucky, this is a regular verb, but remember that when a verb requires a preposition for its object, it keeps that preposition in the passive. Work it out and call this (V).
4 - Other complements: fairly simple here, there is an adverb after the first word of the verb form. Put it there in your final sentence too.
The order is (S) + (AUX - including adverb) + (V) + (A)... Try it for yourself, and submit it if you feel unsure.
No need to delete your post, Argyll... Why should you? It's most helpful, and I agree with you the passive sentence is mostly uh improbable.
Were you referring to the missing "being"? It's safely home now