Outside at local parks, as mentioned above, is not uncommon. It also helps "recruit" others.
Consider, also, finding churches that will rent their facilities rather inexpensively.
Finally, look at the "For Rent" ads. Often you can find a decent-sized industrial space for a good price. As an example, a dozen of us were able to rent a 1200 square foot industrial space for $600/month. We kept that running for three years and it grew into a decent-sized school.
Just some thoughts.
The Professor