Turn Field Label grey? Turn field outline grey? Prevent tabbing into? Prevent editing?
In layout mode, right click the classroom field and select Conditional formatting.
Click the button for a new conditional format and select the Formula option.
Create a formula similar to this, but substitute your table::Field name for the radio button field in place of what I am using:
YourTable::radiobuttonField = "InActive"
Then select a grey fill color for the conditional format.