If it's deterministic, is there a risk of someone just writing a second Expect script (instead of a solution to the programming exercise) that interacts with yours and gives the replies that you want?
I got about 2 assignments into the class before I realized that I was being a chump. I wrote a C program that simply printed out the expected output, and submitted it. I received a passing grade on the assignment, so I took it further. I wrote a Perl script which would submit "int main(void){return 0;}", read the expected output, generate a C file that printed out that output, and submit it again.
Apparently the TA for the class never checked the submitted assignments and relied entirely on the submit program, so I was uncaught for a few weeks. My downfall was trying to make the Perl script as efficient as possible (I had extra time on my hands because I didn't have to do the C assignments!). I had run my Perl script a few dozen times while optimizing it, and apparently the TA received an email every time an assignment was submitted. He brought it up to the professor and I was called into his office. Luckily he was lenient and let me just redo the assignments :)
Early on there was one or two cases where subtle problems came up because the public set and private set accidentally tested things slightly differently, but the TA / prof was quick to respond.