According to "there were 20 participants later, and the ratio of students who participated and did not participate was 3: 4", we can know that there were 3 students who participated and 4 students who did not participate, and the students who participated were 3/(3+4)=3/7 of the total number of students in Grade 6.

Then the corresponding score of 20 people is: 3/7- 1/3.

To find the total number of sixth-grade students is to find the unit "1", divided by the calculation: 20/ (3/7- 1/3)=2 10 students.