As I said earlier, having 5 additional pupils is unusual but it could happen, especially (as prh47bridge points out) if numbers gradually creep up during YR and Y1.
Para 2.15 of the Admissions Code 2012 gives the complete list of exceptions for 2013/14 onwards (the list in the old code is somewhat shorter - I can't immediately find the link but it will turn up via Google, I'm sure):
2.15 Infant class size ? Infant classes (those where the majority of children will reach the age of 5, 6 or 7 during the school year) must not contain more than 30 pupils with a single school teacher. Additional children may be admitted under limited exceptional circumstances. These children will remain an ?excepted pupil? for the time they are in an infant class or until the class numbers fall back to the current infant class size limit. The excepted children are:
a) children admitted outside the normal admissions round with statements of special educational needs specifying a school;
b) looked after children and previously looked after children admitted outside the normal admissions round;
c) children admitted, after initial allocation of places, because of a procedural error made by the admission authority or local authority in the original application process;
d) children admitted after an independent appeals panel upholds an appeal;
e) children who move into the area outside the normal admissions round for whom there is no other available school within reasonable distance;
f) children of UK service personnel admitted outside the normal admissions round;
g) children whose twin or sibling from a multiple birth is admitted otherwise than as an excepted pupil;
h) children with special educational needs who are normally taught in a special educational needs unit attached to the school, or registered at a special school, who attend some infant classes within the mainstream school.
Prh47bridge - Ah, yes, I was forgetting that the one year limit on exceptions was ending. I am assuming, though, that (as with the rest of the Admissions Code 2012), the change to how long a pupil is 'excepted' takes effect for 2013/14 admissions. So, as far as I can see, the excepted pupils at OP's school are only excepted for a year because they were admitted under the old code unless the change applies retrospectively.
Owlelf - I think this is something else you could ask the HT to clarify. Will the additional pupils continue to be 'excepted' after the year in which they were admitted (whether that was YR or Y1)? Whether they are excepted or not, you can ask how the school will ensure that the class size will not have an adverse impact on each pupil's learning.