The object is plural ("years") and in the next sentence, it's singular. I'd probably switch it to something like "She still remembers being in school; it was a pleasant time in her life" or "She still remembers her school years; they were a pleasant part of her life."

Sounds natural to me. Strictly speaking, these two options would be slightly more correct: She still remembers her school years. It was a pleasant time in her life. She still remembers her school years; it was a pleasant time in her life.

I would go for "it was a pleasant period in her life".

She still remembers her school years, as/for/because it was a pleasant time in her life.