Disable all Controls except for New Record Controls on Continuous subform

[padlocked17]

I have a continuous subform that allows for scores to be entered for students. I want to disable all controls to change data for previous entries but leave the new record that a continuous form creates enabled for new entries. I also want to have a button to place on the right hand side to edit each previous "disabled" row if the button is clicked.

I've played around with this but haven't had any success. Any ideas on how to accomplish this?

Thanks!