>>Does this involve brothers & sisters who are not twins ?
>>
>Haven't thought about this possibility.

It's just feasible to get brothers/sisters born less than 12 months apart & so be in the same year.

>
>>If there were also 1 set of quadruplets, then there would be 33 in the class.
>
> Yes, this is another one.
>
>>Or is the answer simpler/more obvious than that
>
>No, the way I saw this question asked, answers 31 and 33 are correct.

Multiple sets of quadruplets would increase numbers in the class too, 2 sets would give 35 in the class & so on. This would be a very well known class.