i got told this because im doing this in my subjects but wanted to know whether its true
It means the school does not think grades 6 through 9 are achievable in that particular subject, and that the student's interests are best served by focusing on aiming for grades up to and including grade 5.
Between 55% and 60% of students take Maths at Foundation Tier, with the remainder at Higher Tier. Does that mean that 55% - 60% of students nationally are "dumb"?
(Note that the table shows the percentage taking higher tier; source)