You can find a business activity code at the IRS listing here: https://taxmap.irs.gov/taxmap/instr/i1040sc-016.htm
I searched "education" and found code #611000 or you can always use the miscellaneous code #999999.
If that was your only income related to this type of work, then yes, you could put $0 other income.
Note: If you were paid for your work, it's considered business income for the IRS. If this is correct, you shouldn't report it as non-self employment income and that sounds like why you are having difficulties.