I highly recommend Philip Bernardi's
Improvisation Starters, published by Better Way Books.
I particularly like one major section where, instead of merely giving students situations to work with, Bernardi provides each actor with an objective that plays against the other actor's objective - much less risk of a scene withering away because no one has any idea of where to go with it.
I found the book in a national chain bookstore and have been using it for years. Most online booksellers have it.
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Jeff Grove
Theatre Teacher, Aesthetics Department Chair
Stanton College Preparatory School
Jacksonville FL