The second one is correct, and Americans still use the subjunctive correctly most of the time, especially in formal writing (keep that in mind if you're taking the SAT, GMAT, or other American university entrance exams). The tendency these days in British English is (sadly) to avoid using the subjunctive mood (so don't worry about it for the IELTS).

Both are correct. As a student, you should learn and use "I wish I were..." It is definitely correct. Native English speakers argue about whether "I wish I was..." is correct or incorrect. In the United States, when writing or speaking carefully, according to the standards of "good English," "I wish I were on holiday" is strictly correct. "I wish I was on holiday" is slightly wrong. However, the English language tradition is that dictionaries simply record the language as it is actually used. "I wish I was..." is so common that you can't really say it is wrong. Grammarians have been complaining about the common use of "I wish I was" for over a century (Mencken complains about it in "The American Language," 1919). The very long usage note under "if" in the American Heritage dictionary says: "With all deference to the traditional rules, it should be noted that a survey of the prose of reputable writers over the past 200 years would reveal a persistent tendency to use the indicative _was_ where the traditional rule would require the subjunctive _were._ A sentence beginning If I was the only boy in the world, while not strictly correct, is wholly unremarkable."

I wish I WERE…

The second one sounds correct to me; the first one not. I am not a grammarian, though.