The first school sends 3 times as many German students as French students, so if they send 21 German students, they must also be sending 21/3 = 7 French students. The first school then sends a total of 21 + 7 = 28 students.
The other 3 schools send the same number of students. There are 4 schools, so the total number of students is 4 × 28 = 112.