Answer: 50 students. Write the numbers of boys and girls as 7k and 3k. Because both numbers must be integers and 7 and 3 are coprime, k must be an integer. The class therefore has 10k students.
Among 50, 37, 73 and 21, only 50 is a multiple of 10. It gives 35 boys and 15 girls, whose ratio is exactly 7:3.