Compilation without implementation source

For an educational exercise using GNAT and gnatmake, I need to hide
implementations of some modules from students in the simplest possible
way.

Is there a way to give them .ali, .ads, and .o files while withholding
the corresponding .adb, somehow telling gnatmake (and gnatbind) to
ignore the absence?

Or must I provide a library?

Other methods?

Many thanks for your help.
.