Well, I think given the same situation, most girls would feel the exact same way you are feeling. In my opinion this is a Big. Red. Flag.
Worst case scenario....... he likes this girl and is either already dating her at uni or would like to date her---------- best case scenario....... she really is just a platonic friend and he isn't respecting you enough to ask you to his ball (even after you said you would like to go with him) - either way it's not good. You are his girlfriend and asking him not to escort (or invite) another girl to a ball is not putting unnecessary restrictions on him. I don't think you're being paranoid you don't know what his 'friendship' status really is with that girl, meaning she actually could just be a friend, but what you do know is that he didn't ask you, his girlfriend, and was acting dismissive of your feelings when he should have been reassuring you if it was really nothing. He genuinely may not think it is a big deal at all and in his mind it really isn't but once you expressed your (very rational) concern he should have respected that. He should have welcomed the fact that you were willing to go with him. I would definitely be having another conversation about this (or 2 or 3 more) with him. If he's still dismissive and making you feel like you are in the wrong w/out giving a legitimate reason why taking her is the better alternative to taking you (and a reason that you can understand and agree with) then, honestly, you should reevaluate this relationship and it's future. You do not want to become the disrespected push-over girlfriend. Trust me it is better to be single waiting for the right guy than to be the disrespected push-over girl. Best of Luck to you!