Miss Cross dancing with Max Fischer at the end

I believe their dance simply represents Max's ascension to adulthood and constitutes a beautiful and poignant act of remembrance.

TotalFilm actually listed it as one of their top 20 film endings ever, commenting:

The Ending: His play a triumph, and an imperfect but stable compromise reached with Ms Cross, Max Fischer looks ahead to young adulthood with a new girl on his arm and a new perspective on his own myriad vagaries.

It's been a rocky ride for the young firebrand, but the sense of emotional achievement is palpable.

Goosebump Moment: Right as those "all the world's a stage" curtains swish closed, the chorus lyric to the Faces' Ooh La La kicks in over the top, and Wes Anderson's hitherto-somewhat-obfuscated point suddenly comes into beautifully sharp focus.

If They'd Gone Bleak: Max would've tried to give Ms Cross the reacharound, had his specs slapped off, and left us with the unimaginably sour implication that he'd learned absolutely nothing about the world or his place in it. Gah.

As their comments show, the bleak finish would have involved Max descending into his childish manner again, making an inappropriate move and thus showing nothing had changed in the film. Instead, regardless of what his true feelings are, Max contains himself and we are given a final reminder of his old life before his journey into the "new".

Rolling Stone magazine agree, stating:

To call Rushmore a romantic triangle about clinical depressives doesn't allow for the film's bracing humanism. No tidy happy ending here. Just a cotillion honoring Max's Vietnam play and allowing the major characters to come together, change partners and dance to a Faces song, "Ooh La La," that links youth and experience in a lovely, fleeting moment of reconciliation before the shooting recommences. Anderson closes the curtain on his movie as if he were directing a play by Max Fischer, which, of course, is just what he has done. Bravo.

So whilst there could be many interpretations, I feel the ending is simply the first snapshot of Max's adulthood.

I think the ending is representative of what was previously thought to be ideal which actually isn't reality and this is neither a bad nor good ending it's just realistic.

Herman Blume earlier on in the film comments that he will end up with a very young woman who is chinese and does in the ending for only a dance as this is not what would best suit him and Max dances with Ms Cross who he previously thought would be the love of his life who isn't or shall not be.

The ending in this way is good as both Herman and Max would probably realise what they really have interested in them of which is the most suitable and long-lasting.