I would have to agree that adding a pedestrian crossing behind the school would make sense. Behind the school, the tracks are up on an embankment that separates the school from a large residential area. Adding a pedestrian tunnel under the tracks like they have at Winchester High School would definately make things safer. If you look at the satellite photo I posted below, you'll see that the school is dead center between the Concord Ave underpass and the Brighton Street grade crossing. What's interesting is that directly across the tracks from the football field is a vehicular access road (labeled on Google Earth as Alexander Ave) which runs from the street directly to the tracks and dead ends (with an obvious foot path on the school side of the tracks).
In this situation, imagine you're a high school student who doesn't yet have a car and you live in one of those houses directly across the tracks. I measured it using the ruler in Google Earth, the white house at the interesection directly across the tracks from the school building is roughly 500 feet from the school building, yet to legally walk from that house to the school requires a 1.14 mile walk via the Concord Ave underpass. Like any other smart and responsible railfan, I stay off the tracks at all costs, but if I was leaving for school on foot on a cold snowy morning, knowing that I was about to walk over a mile to get to a building I could see from my house only 500 feet away, I don't think I'd be able to resist the urge to just walk across the tracks. This is from me, a railfan. Now imagine the mind of a non-railfan in the same scenario, someone who roughly understands the dangers but knows hardly anything about the illegality of such a crossing, and you can see why it would be so tempting. The students in this scenario aren't on the tracks to be reckless, they're simply trying to avoid a ridiculously long walk to avoid, ironically, a public transportation system.
Luckilly for BHS students, the Belmont CR station is barely beyond Concord Ave, so non-express trains will be running slower than usual at that location, and freight on this line is currently very rare (if any anymore), but something still needs to be done. Like I mentioned, Winchester High School has a pedestrian tunnel under the Lowell Line, it's there to provide off-street access to the athletic fields and is only a few hundred feet from a road underpass. I wonder if this would be something the town (and MBTA) would put into consideration.
Trains aren't dangerous, it's lack of common sense that's dangerous.