Let's see, my band now (Cog) is from the Ateneo batches that had: Plane Divides The Sky, Boy Elroy (not sure if they had high school AHS members, though), Chicosci, Dicta License, Kjwan, Happy Meals, Greyhoundz (via Niņo Avenido), Out of Body Special, Sponge Cola, Rampqueen, 13 Needles, and Day One Movement.
when I myself graduated from the high school, there were bands like the Happy Shrimps and Under Blue Skies (I think atenean ito). I later got into a band with Dale Ibay (Cookies for Kids) for a short while.
That's a lot of bands, and if I were to think like the classic Arrneean, I'd think, shet, dami yan! talo ang Lasalle.
But I'm not. that's a lot of bands, yes, but i have no idea, actually, of how many bands have come out from Lasalle - I never really kept track. It's possible that they have more bands out there, and I am inclined to think that they have more solo singers coming out of their alma mater than Ateneo does.
So should there be a rivalry? I think not. it's all music and musicians anyway - that ateneo/lasalle rivalry crap is just that: crap.