Grammar exercise helps individuals to learn the correct form of English. The given sentence can be written as "Leila has been a nurse since 1999" and in second sentence "we haven't met since our last year at high school".
The word since is generally used with point of time in the past.
The given sentence is showing a point in the life of the given persons.
Thus, the sentence can be written by using since as "Leila has been a nurse since 1999" and in second sentence "we haven't met since our last year at high school".
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