he's your friend right? You love him? Tell him the locker thing made you mad and next time a heads up would be cool, cause it's your locker too...tell him you thought the gesture for her was sweet, but to you it was a pain cause you needed to put your stuff up. If he is YOUR friend, he will understand and not do that again...
As far as him ditching you(and the group) for the girl, give it some time...when a relationship is new, you tend to spend all your time with that someone wonderful, eventually it will cool down...why don't you invite her to hang out with you? You said she was nice, so get to know her too, plan something you all can do as a group and make sure she feels welcome.
In the end, be gentle with your honesty, but do be honest...tell your friend you miss him but that you are glad he is "in love" or whatever it is...if that isn't something you can do, then do try to hang out with his new girl.