My son took SAT first time in 8th grade (I think) for CTD Talent Search. Then he took it again in junior year, for National Merit purposes. Testing in senior year was mainly on SAT II’s and AP’s, but he also took the ACT once then. This all pretty much rolled off his back, since he had no test anxiety and didn’t do any special prep for the exams aside from looking at the general instruction booklet. His scores were always exceptional (total over 3900 for 5 tests of SAT I and II).
My daughter deferred taking SAT for first time til senior year and was disappointed, so repeated them with only small improvement of scores. She also took ACT once in senior year. Her SAT and ACT were good scores, especially considering that she did no prep at all and in any event she was aiming to attend art school. For her, prep of her portfolio was far more important than test scores, which had to be good (which they were) but not outstanding.