>Is 1:1 the best rule for coverage?
If you have students using lots of devices - yes.
As a general guide, 1 AP per 10 students will provide excellent coverage, 1 AP per 20 students will provide good coverage.
>I would love to try turning off 2.4ghz, I just don't want to boot anyone off who needs it
This is the actual choice you are making - kicking off a small percentage of devices so that the vast majority will get a greatly improved result.