I disagree with your English teacher.
For possibilities and things that are not true, technically one should always use "were" (e.g. "If I were a gameboy" or "If he were taller"). In my opinion, it does not matter if it could or couldn't become true.
"Was" is just the past tense of "to be", as you know. ("I was here yesterday.") However, it is frequently used instead of "were" for these non-true statements — "If I was a gameboy" or "If he was taller". This is technically incorrect, but as Cynthia said, it is very common. I have seen it argued that it is becoming accepted usage.