St. Mary’s County public school administrators figure that they are going to need four new schools in the next decade to keep up with enrollment increases.

That’s three new elementary schools and a new high school. So far the only one mapped out is a new elementary school on the outskirts of Leonardtown. Construction hasn’t started yet, but the current timetable calls for it to open in August 2015.

When it does open it will be immediately filled; all but three of the existing elementary schools in St. Mary’s are already over capacity, several of them by more than 100 students.

It’s no real surprise that enrollment projections are jumping up, and forcing an accelerated timetable to build new schools. St. Mary’s County grew at a faster rate than anywhere else in Maryland during the last decade, according to the U.S. Census Bureau, and last year ours was the ninth-fastest growing community of its size and type in the United States.

Calvert and Charles counties, where population has also grown rapidly, have already had school building sprees. St. Mary’s has built some replacements for aging schools in the past two decades, but has opened just one additional public school in the past 30 years — Evergreen Elementary in Wildewood, which opened three years ago and next year is expected to have as many as 825 students; it was built to hold 646.

St. Mary’s County hasn’t built a new public high school since 1978, when Leonardtown High School opened.

Instead, and really at the urging of the state government, which funds a big chunk of school construction in Maryland, St. Mary’s has made do with expanding existing schools — additions were built at all three high schools in 1997, 2000 and 2002.

But let’s be clear-eyed about this ambitious plan to build new schools. It may not all happen in the next 10 years. As a practical matter the construction schedule is likely to depend more on when state and local construction money is available than how much enrollment spikes.

So what will happen if the timetable lags as schools become increasingly crowded? The same thing that is happening already. More classroom trailers will be parked outside of existing school buildings.

There is no way to avoid this. The state doesn’t seriously consider requests for new school funding until there are already enough students jammed in elsewhere to fill the yet-to-be-built school.

Classroom trailers don’t equal an inferior education. There are some teachers who even prefer them, because there are fewer outside distractions for the students. But there are limits, for example, to how many students can be served in a school cafeteria at something approximating a lunch time. Enrolling more than 800 students in one elementary school pushes those limits.

Four new schools in 10 years? We’ll see. But attention must be paid, planning must proceed, the search for places to build these schools and the push for funding must continue. This is one of the obligations of a prosperous, growing community.