When you have to count in how many ways you can arrange something, you need permutations: you have 9 possible students to schedule as first, the second visit will be scheduled with one of the remaining 8 students, the third one with one of the remaining 7 students, and so on until the last one, therefore:
9 × 8 × 7 × 6 × 5 × 4 × 3 × 2 × 1 = 362880
The students can be scheduled in 362880 ways.
