Hmm, a quick check...
If I didn't miscount, since 1990, there have been 288 deaths in "school shootings" (note that that number includes at least one case of an adult shot by another adult in a school parking lot).
With 330 million or so people, and 35 years, that puts the annual per capita death rate from "school shootings" at 0.000000016.
Terrorist attacks in the same 25 year period. Hmm, let's use 9/11 and call it good. 2753 from the direct attack. So, 0.00000033.
Yep, looks like you're right.
Even with a very strict definition of "terrorist acts" (essentially reducing it to "there's been ONE terrorist act"), and a loose enough definition of "school shooting" to include adults unrelated to the school in any way shooting each other in the parking lot, we have about ten times as many terrorist deaths as school shooting deaths in a generation.