You could use past simple or past perfect, as people have said. The reason English has both is to clarify the timeline. In your example, it is pretty clear in either tense. "I studied in the school for two years before I left." or "I had studied in the school for two years before I left." This is because the events happened in the order they were mentioned. The combination of past simple and past perfect allows you to change the order of the events. For example, "I left school. I had studied there for two years." is still correct. Past perfect lets us know that the action of studying occurred before the action of leaving. If you said "I left school. I studied there for two years." people might think that you went back for two more years after you left. The combination of these two tenses is especially useful because it allows you to talk about events more flexibly without confusing the listener. It is especially useful for telling stories.

You both are native English speakers.

Why does English have different tenses if you tell me that I should use Past Simple for discribe a period of time?

Okay Greg! Could you please remake the sentence but you should use "2 years" and "before I left"? Don't use Past Simple.

Simple past tense can be used for periods of time from the past. I agree with Jack - it's the best choice here. "I studied in the school for 2 years before I left it". A similar example using the simple past tense is: "I taught at the school for 3 years while I was there."