How long can she go to school?!

I have 2 children with my partner. He has 2 children with his ex. He pays child support and for the last 8 years has been paying 100% of extra expenses on top. The reason he pays 100% of the "extras" (including 100% of flights to see his kids after the ex moved accross the country) is that she was under-employed for 3 years, and now has been "upgrading" her education for the last 5 years. She already had marketable skills, but it appears she doesn't like working full time (even though her kids are teens). We cannot continue to pay 100% of extra expenses and orthodontic work just got added on. Our 2 toddlers get NOTHING and we are in DEBT every month. We are told we have to pay as she has NO income, but when does it stop?!?! What can we do?

Who tells you you have to pay?

You are only obligate to pay what the court order say you are obligated to pay.

If the mother moved across country then petition the court to get her to pony up for travel expenses. You would have been more likely to succeed in this if you had petitioned immediately after she moved, but you still have a chance.
Alternatively, you could petition to have support reduced to compensate for travel costs.

What do your court order say about medical expenses? The standard in my State is that the non-custodial parent pays one half of all medical expenses over $100. It is the responsibility of the custodial parent to pay the bill, and then provide documentation to the non-custodial parent in order to be reimbursed.

Our Lawyer/Divorce Agreement tells us we have to pay. We live in Canada and any "extra expenses" are to be split according to income, so if he makes 50K and she makes 50K then expenses are split 50/50. Because she has ZERO income our lawyer told us we have to pay 100% of the extras but I feel like the ex is really working the system and getting away with it. Not sure how to change our divorce agreement, especially with a lawyer that tells us "that's just the way it is". I know we should get a new lawyer but we can't afford one right now. Is there anything else we can do?

In the US (or Ohio, at least), financial responsibilities are divided according to the earning potential of the parties, not their actual earnings. So, quiting your job or not working doesn't get you off the hook. Even someone who has never held a job is presumed to be able to make a default income, usually around $15,000, for purposes of calculating child support.
Check the laws of your province in Canada. They are probably available on a website.