I remember as a kid in the late eighties, I used to watch a movie on
HBO all the time.  It was a movie from what I remember, has something
to do with possible robbers or other criminals on the run from the law
where they may have hid out in a small rural grade school.  The men (3
or 4) would wear masks of maybe Nixon and Santa Claus.  The teacher
took the students and escaped and tried to run from them.  Sorry for
the lack of details, but I do remember, however demented, that the
kids were trying to swim through an underwater cave, and used this
small pocket of air to make it, since it was too long to make it on
one breath.  Another detail I remeber was that the masked men found a
house "out in the distance" with a light on and went to that house to
eventually blow the crap out of them with their shotguns.  It wasn't
all that popular or financially successful I don't think.  I remember
maybe seeing this movie all of the time because it was always on with
movies like Red Dawn, Gleaming the Cube, and Summer Camp Nightmare.

I am certain that you are remembering "Fortress," in which a teacher
and some of her young students are abducted from a school in the
Australian outback and terrorized by brutal masked criminals. This
movie has been shown on HBO more times than I can count.

Internet Movie Database: Fortress (1986)
http://imdb.com/title/tt0091069/

Here are some online descriptions to help verify that this is the correct movie:

"This movie is about a teacher (Rachel Ward) who gets abducted along
with her class by a group of men wearing various face masks. They are
taken into the outback and sealed in an underground cave. The teacher
and her students begin searching for a way out and discover they must
swim for their freedom. Once free, they once again become prisoner of
those who abducted them."

Amazon: Fortress (1985)
http://www.amazon.com/exec/obidos/tg/detail/-/B00000F3MC/104-8867107-6979939?v=glance

"Sally Jones and her class are looking forward to the few remaining
days to the end of term. Pleasant anticipation turns to stark horror
as four grotesquely masked men descend on Sally and the children and
whisk them away at gunpoint. A damp and dark cave becomes prison for
Sally and her nine young charges until they manage to grope their way
to freedom by way of an underwater exit. Their newfound freedom,
however, is short-lived as their captors are waiting for them at a
nearby farmhouse, where they have overpowered the old farmer and his
wife."

Crawfords Australia: Fortress
http://www.crawfords.com.au/libary/movie/fortress.shtml 

My Google search strategy:

Google Web Search: fortress "rachel ward" cave masks
://www.google.com/search?hl=en&q=fortress+%22rachel+ward%22+cave+masks

I hope this is the right movie! If it is not, please request
clarification; I'll be glad to offer further assistance before you
rate my answer.

Best regards,
pinkfreud