When Zoe Portman finished her essay for the Common Application the widely used college-admissions form that can be sent to multiple schools she thought she had completed the bulk of the application process.

But the senior at Harriton High School in Rosemont, Pa., who is using the Common Application to apply to 10 schools, was in for a surprise: the supplementary forms that nine out of the 10 schools require along with the Common Application. It looked easy at first, Ms. Portman says, but then “you realize there’s question after question that you have to answer.”

By the time she finishes it all, she says, she will have given written responses describing everything from a risk she once had to take to the place where she grew up. “It’s such a hassle,” she says.

The Common Application was introduced in 1975 to streamline the admissions process for students, but as the experiences of Ms. Portman and countless others this college-application season show, it has evolved into something far from simple.

The supplements have become more demanding, creating headaches for applicants, as an increasing number of colleges particularly more-selective institutions such as Stanford University and Northwestern University have started accepting the Common Application. At the same time, applicants and counselors have bristled at attempts by Common Application Inc., the nonprofit group that supplies the form, to prohibit students from tweaking their essays and other parts of the application for different colleges. The frustration has helped to create an opening for a new rival, the Universal College Application, which arrived this year and is already accepted by more than 50 schools.

The Common Application board used to require that a school’s supplement be limited to one page. But it changed that policy in 1995, as more highly selective institutions that said they needed to ask specific questions of their applicant pools began to join the consortium.