Callings over

Callings over -- from Darwin's autobiography; "I very often ran there [home] in the longer intervals between the callings over [something that happened at 'school in Shrewsbury'] and before locking up at night."

What are "callings over"?

From the Oxford English Dictionary, I find that "calling-over" is a term used in British schools for what's known in the U.S. as roll call.