No, it’s not exponentially higher. It’s somewhat higher, how much is unclear, but I’m not sure you should feel guilty about it.
Schools don’t seem to be the hotbed of transmission that one would think they should be, given their importance in pandemics of the past, lack of social distancing and hygiene of the population. 
High schools may be more of a risk than junior high or grade school because of the age of the students.
We talked a lot about this in this thread-. If you scroll through, you should find some references, Masks on kids - #24 by QwertySC I’m guessing your school district made some of their choices based on the evidence there is, so far, on kids & COVID19.
My DH is a doctor in primary care, so he’s in close contact with people who may have the virus- even clinical screening for it will miss about a third to half of people with it. Other people in clinical settings will have far higher risk than that- an anesthesiologist friend of mine doesn’t even go home for weeks at a time due to the risk.
So risk is definitely relative. I’d worry more about children at my house than yours. I myself am immunosuppressed, so this is something I think about. We can go on worrying about every possibility or we can try to educate ourselves as to the actual risk. It’s hard when this is a potentially serious issue that has never come up before in our lifetimes, and we’re used to (at least believing) we’re safe and in control.
Personally, because of your concern, I think you’ll take every precaution possible, and my kids could go to your house anytime.