Can my wife use our internet service as a deduction since she's a full time online high school teacher?

Yes, she can deduct the internet costs for an online teacher but must first apportion the expense between personal and business use.

Not if she is a W-2 employee.   The Tax Cuts and Jobs Act eliminated ALL unreimbursed employee business expense deductions for 2018-2025.

Side note on wifi use to previous answer: 

If you haven't already claimed educator expenses for  the out of pocket allowed of $250 per individual then do it there.  Wish this was more for teachers - thank her from us parents. ❤️ I'd like to see this amount increased bc teachers need it.  

 See this IRS tax topic link on expenses for educators

search this

irs.gov/taxtopics/tc458