If you think high school seniors are lazily waiting until after Christmas to apply to college, think again.

October and November have become crunch time for the growing number of students taking advantage of the early deadlines offered by some colleges that hope to fill part of their freshman class before the spring onslaught of applications.

For students, applying early can mean a better chance of getting into their first-choice college and peace of mind for the rest of their senior year.

But it can also mean navigating a maze of deadlines and requirements well before most seniors even have started an...