According to Webster's not much. I'd say they were synonyms.
Shoal: a large group or number : CROWD <a shoal of fish>
School: a large number of fish or aquatic animals of one kind swimming together
http://www.m-w.com/dictionary.htm
Shoal: a large group or number : CROWD <a shoal of fish>
School: a large number of fish or aquatic animals of one kind swimming together
http://www.m-w.com/dictionary.htm