Which is correct?

They have made my high school experience unforgettable.

Or

They made my high school experience unforgettable.

Both are correct.

Depends on the tense in which the sentence has to be used.

THE 2ND ONE

Both sentences are grammatically correct. However, the choice between the present perfect tense ("have made") and the simple past tense ("made") depends on the context and the intended meaning.

1) "They have made my high school experience unforgettable" suggests that the action of making your high school experience unforgettable started in the past and has continued up until the present. It emphasizes the ongoing impact of their actions on your experience.

2) "They made my high school experience unforgettable" indicates that the action of making your high school experience unforgettable happened in the past and is complete. It focuses on the specific event or period of time when they made your experience unforgettable.

To determine which one is more appropriate, consider the context and what you want to convey. If you want to emphasize the ongoing impact and relevance of their actions, use the present perfect tense. If you want to highlight a specific moment or period in the past, use the simple past tense.