A Batman animated series featuring everyone as teenagers attending the same school? It almost happened in the shape of "Gotham High".
Artists Jeffrey Thomas and Celeste Green made a poster of the Joker and Harley Quinn as high school students in the same school. They sent the image to DC [below] who went on to hire the pair to develop their "Gotham High" as an animated series. Unfortunately it died in development.
Thomas & Green describe the idea as such:
"We all go through incredible changes as teenagers: growth spurts, bad skin, a sudden insatiable need to uphold justice and avenge your murdered parents.... Well, that is if you're Bruce Wayne. As if being a freshman at Gotham High wasn't tough enough, Bruce's insomnia and technological fascinations are taking their toll. Instead of spending his time studying, he has begun to obsess over an emerging personality trait: Batman. But under the watchful eye of his guardian and steward, Alfred Pennyworth, Bruce is forced to put his intelligence to good use: graduating high school. But given his classmates, can Bruce survive Gotham High?"