Lost childhood book

I’m trying to identify a middle grade/young teen book series from the early-mid 1990s (I probably read it around 1995).
It was NOT one of the major series like Babysitters Club, Sweet Valley, etc. It was a shorter series — I think only 4 books.
Premise:
Four teenage girls from the same school win a writing contest and get to go on summer trips with a young “cool” male teacher and a female adult assistant/chaperone (who was also his romantic interest throughout the series).
The books were mystery/adventure with light romance.
Things I remember:
The first book took place at the Grand Canyon and involved rafting/hiking

One of the girls had a Native American love interest in that book

Other books took place in Maine, the Greek islands, and Spain

One girl’s name may have been Mallory

The cast was diverse:
one girl was African American

one was Asian

one spoke Spanish and French

Two of the girls were already friends before the contest

The books were paperback originals, probably from Scholastic/Point/Bantam/Avon era school book fairs

I vaguely remember a sailboat on at least one cover (possibly the main series image)

Does this ring a bell for anyone? This has been driving me crazy for YEARS.