That's funny, the teacher isn't economist in this case but only blind hate towards bitcoin.
Luckily, in my country, economic teacher in school don't bother talk about bitcoin because they think bitcoin isn't part of economy.
1. That's illegal : He did not bring many proves here, just saying that since it has not been regulated as a currency, we were in a grey area, and something in a grey area is rarely something that will become totally white from one day to another. He added that the sole purpose of that was to buy all kind of drugs over the Internet.
If government don't say bitcoin is illegal, then bitcoin is still legal for very limited usage and bitcoin can be used for almost anything just like fiat.
The most popular example are video games on Steam and software from Microsoft.
2. It will crash : He said that he had studied Bitcoin's price evolution throught the time (what I doubt) and said her that being so constantly rising was a sign that it was not a viable project. Its proof : manipulated prices of things like silver or wheat are very stable !
Might be true, but bitcoin has lots of potential and could be global currency someday.
3. Anyone clear in its head wouldn't use it : First, he doesn't recognise any kind of manipulation of the money and the danger that there is for people keeping all their savings in fiat. I know that because I had him as a teacher too some times ago. So trying to explaining him about the fact that there is a clear war against cash and evident facts, like that that shows our money need to be converted into something viable to be protected from massive surveillance, is useless. Him not understanding that, since its the best argument I have, I couldn't have replied. When she told him about the speed of transactions, he replied "we have PayPal", so I said me that it wasn't worth to ever try explaining anything. As a final point, he said again that the sole real use that is making standing apart is that you can buy drugs on the Internet...
I can't argue with this if the teacher don't understand how bitcoin (or blockchain) works. Maybe you should give her the whitepaper of bitcoin first.
4. Bitcoin at a bigger scale would fail : I couldn't explain you much what he said because what she told me wasn't very clear so I personnally did not understood, as her I presume how confuse her explaination was. To summarize, he said that it's still a little market and that it will fail to scale and will go back to close to nothing in value.
If bigger scales means bitcoin as global currency, i'm sure it won't fail as long as there are hundreds of fast nodes and block size limit has been removed.