There are many cases in the news of accidents with sometimes a large number of party balloons filled with hydrogen or other flammable gases.
One of the larger episodes was in 2012 in Armenia, where thousands of balloons exploded during a meeting, injuring 154 people, of which 4 seriously (the video is of poor quality): https://www.youtube.com/watch?v=jWEm2sS7Dw8
a party balloon - say a cubic foot - is about 2g of hydrogen. Involves 16g of oxygen. So we're talking 18g of very fast burning, borderline detonating mass. Releases 240 KJ of energy.
To compare the hand grenade - 60g TNT https://en.wikipedia.org/wiki/F-1_grenade_(Russia) - releases the same 240 KJ of energy.
I had a science teacher that did this in class, then taped a match on the end of a yardstick and held it under the balloon. They made quite a bang. I wouldn't want to be right next to it when it went off.
While hydrogen-air mixes explode really readily (outstandingly wide flammability and pretty wide detonation range), and the energy released is considerable for the weight, the actual explosion does not produce a particularly high overpressure wave.
That's because the starting density of the hydrogen air mixes at near atmospheric pressure (such as in a balloon) is pretty low. Also, the balloon does not significantly contain the explosion, which reduces the danger a lot. I would not want to do it in a glass container.
My chemistry teacher told us how once when he ignited helium in a test tube, the tube broke and he ended up with pieces of glass embedded in his skin. The students had face masks and he was looking the other way "just in case" for this "safe" experiment but he could have easily been blinded.
Things can always go wrong. We probably shouldn't strive for 100% safety because they we'd spend our lives in a padded cell. But we also shouldn't assume things are safe because they're common or routine.
Along with the other commenter, I'll add that a classroom is usually a lot bigger than a home dining room or other domestic party locations. That size also helps things dissipate instead of reflect. Not sure by how much but I'm sure it does something.
We did this. One balloon with plain air. One with pure hydrogen. One with 50/50 hydrogen and air. The one with pure hydrogen popped closer in magnitude to the pure air than it was to the 50/50 mix.
ETA: I may be misremembering, the more I think about it, the more I recall that we did not use air, we did use pure oxygen. Not like it was hard to get (and we had lots more interesting stuff than that in the lab, this was the 80s...). But the outcome I do remember. The entire point of the experiment was to examine the difference between the individual pure elements and the mix. We expected the pure hydrogen to be far more interesting than it turned out.
Pure hydrogen in a balloon produces a low, loud, very satisfying bang. Completely different from a sound of an air balloon popping. Here is a video from a very good Royal Society of Chemistry demonstration series on various unusual combustion process:
Hydrogen mixed with air or with oxygen produces an ear piercing supersonic detonation, exceedingly loud and unpleasant. Not recommended for demonstrations.
As a kid I took a lot of classes at the Lawrence Hall of Science in Berkeley, which was paradise for fledgling nerds. On the last day they would have a little closing ceremony with some cute little science experiment. One of my favorites was "Going Out With A Bang".
The instructors would bring out a helium balloon and a candle on a meter stick. The balloon goes pop, huzzah.
Then the twist. "Hey, wanna do it again?" All the kids would be like "meh, I guess?" They would then bring out a balloon full of hydrogen (maybe some oxygen too?). It would look identical to the first one, floating there tethered to the lab bench.
When the candle hit the second one, it made a white flash and a really sharp BANG. It was an order of magnitude louder, and you could hear the transient bouncing off the walls and echoing in the halls. It made an impression.
How dangerous are party balloons filled with hydrogen? Not a whole balloon arch obviously.