Personally, I feel that a suit for this kind of job (ie. Summer job), is a bit too much. If you are happy to wear the suit, then thats fine. You wont be given minus points for making the effort.
If it were me though, I would simply wear a shirt and tie, especially as it is the Summer, hence, a jacket might get too hot.
Its highly unlikely that you will be marked down for not wearing a jacket, however, you should look neat and presentable - ie. nice clean trousers/shirt/tie and clean shoes. You will be marked down if you look scruffy/dirty/filthy. Dont get too hung up on this suit business. If you feel more comfortable with the jacket on, then wear it.
My belief is that they will be looking at other aspects - your behaviour, your speech, the way you behave/react to the other candidates during the group interview. The key is to remain confident in your speech (if you are very young, usually, this is where you will fall over a little, as age usually brings an aire of confidence) and come over as someone who has thought about the position he is applying for.
I once went to a group interview for a similar position and one of the candidates was so nervous, he couldnt speak...yes, I really do mean this, he got so nervous, he lost his power of speech. After all of us started talking though, so did he. He was wearing a suit, but I'm pretty sure he didnt get offered the job. Myself, I didnt wear a jacket as it was the Summer - I was offered the job, but accepted a job elsewhere (for more money).
Also, try not to put too much pressure on yourself. Just try your best. This way, you wont be too nervous.
Good luck man.