Not exactly. The reason no article is needed is that the speaker does not want to use an indefinite noun phrase.
I don't wish to make this any more difficult than it already is, so I'll just tell you not to use articles before percentages, although it is conceivable that you may have a reason to, in the right context.
Furthermore, you shouldn't really think of percentages as grades at all. They are not meant to be thought of as discrete grades in the same ways as alphabetical or numerical grades, such as A/B/C/D/ or 1/2/3/4/5.
I do see what you are saying. I am just getting a little confused here. I would like to confirm.
You are saying 70% is a threshold, so you do NOT need an article because it's not a discrete grade (and never should be treated as such).
If that's the case, sentence A (NO ARTICLE) is the correct form, is it not?
(A) Students must get seventy percent or higher on all tests.
I think I got confused because you said B (with an article) is the right answer initially.
Thanks for being patient.