An explosion in applications to DePaul University in Chicago and a push to make more room for freshmen in campus residence halls has led the school to boot juniors and seniors from student housing. A 16 percent increase in the number of applications--to a record 12,031--has the school bracing for a much larger freshman class next fall. DePaul officials want to guarantee limited on-campus beds to new students. Although many students commute to campus, the school has only 2,800 beds in its residence halls, and sophomores need room, too. So many upperclassmen will be out of luck.