I agree, that’s a great question. You’re doing a great job if students are asking questions like that! I’m not a plant physiologist, but here’s my guess about what’s going on. It sounds like you already explained that when a plant isn’t getting light, it stops making chlorophyll. Chlorophyll breaks down, so the yellow (xanthophyll) and orange (carotene) pigments are more visible.

The pigment breakdown happens even faster in sunlight (just like cloth will fade in the sun). That explains why we have to move the lawn furniture around.

Sometimes the grass gets all brown and dry in the fall, but it depends on your climate and the type of grass. This is senescence, where the top of the plant seems to die, but it’s really more like the leaves falling off a tree.

The underground part of the grass plant isn’t dead at all. So what triggers senescence? Daylength and temperature, plus the genetics of the plant.

The plant stops growing well before the first freeze. A “killing freeze” or “hard freeze” (25 Fahrenheit or -4 Centigrade for at least 4 hours) will not really kill the plant, but will destroy cells above the ground. Ice crystals poke through the cell walls, killing the cells, which will then dry out and turn brown. A frost (with the temperatures just below freezing) doesn’t damage the cells unless someone walks on the frosty grass.

My guess is that it’s all about timing. Say the fall has been reasonably sunny and not too cold. There may have been a frost, but not a killing freeze, so senescence hasn’t happened. Then the snow falls, covering the green grass. Now the plant isn’t making more chlorophyll, but the light is also not breaking it down. Freezing may even help to preserve the chlorophyll that’s there, just like spinach in the freezer (total guess). Snow is a good insulator, so maybe the snow-covered grass didn’t even get cold enough to break down the cells and the grass will not even turn yellow after it has thawed out.

I hope this is helpful. It might give you some experiments to try with your kids. Let me know what you find out!

It's because the grasses that live in environments that get snow are merely dormant, not actually dead, while the grasses under a rock can't get light and so die. Also, it's usually only brief snows that have green grass under them - if it's a long snow season, those grasses will dry out and die, too (although the roots underneath the ground are still alive, and will put up more green leaves when the snow melts).

I haven't noticed whether or not grass is green under the snow. If it's green, it's getting enough light [and heat?] to carry out photosynthesis.

Here's a poem about green grass under the snow, but that's not science, so I looked farther.

poem

The link below says that for the most part grass will survive its winter die back under the snow. However, a blanket of snow or coat of frost does make a lawn more susceptible to smother or trample damage. Anywhere toys, tools, sprinklers or whatever other debris is left on a lawn over the winter, one can expect to find a brown spot beneath. Be sure to have a lawn clean and clear before the snow falls, or as soon after as possible. The same goes for trample damage- your lawn is sensitive to trails being stomped out during the winter, so keep the on-lawn traffic to a minimum.
read here

Here's a link about yellow grass [and snow]. Yellow grass isn't getting enough light:
yellow grass

Thanks for teaching kids science. That's valuable.

There are a few things I can think of, but I do not think this is a complete answer, just references. Rocks tend to block both moisture and sunlight, so grass under a rock would probably grow poorly because it lacks two of the most essential plant nutrients. However, snow would at least provide some moisture to the grass underneath, and it may also provide some insulation from heat loss. My hypothesis is that grass under snow is "watered" by the moisture from the snow and partially protected from the cold air by the insulation.