First of all, why do you need work authorization work to your in own college? I have shown my Tax documents in my case for I-485. I used to be a mentor during my study. I guess your lawyer is simply confused!!

if your lawyer don't want you to include then don't include. Worst case is you might get an RFE and you can simply respond it with that document. USCIS won't go after IRS to pull the details. I have even seen people who worked illegally in gas stations and got approved. So don't worry man you are more than safe!!