Both are correct IMHO. The teacher that said 2 is correct, before the advent of computers with variable width characters all characters were the same width. Putting 2 spaces after a sentence made it easier to tell when the senetence ended.
The teacher that says 1 space for web design is correct because all white space is reduced to 1 character. White space includes space, formfeed, newline, carriage return, tab and vertical tab. By only using one space instead of two you have reduced the amount of information that needs to be transmitted and the result is the same. The reduction in characters transmitted may not make any difference to people with fast Internet access but to people who have slow access it does. Also if you are the one paying to transmit the data you would prefer to pay for less than more. Since the result is the same on the web I would suggest less is better.