You teacher is right - we use "I were" for imagined situations as well as past plural. It's the same for all persons: I wish I were, I wish you were, I wish he were, I wish she were, I wish it were...
Although you can find many examples of "I wish I was", you also need to know "I wish I were" is perfectly correct and still in use. The grammar rule is that subjunctive mood follows "I wish", and "were" is the subjunctive form of "to be".