There is no future tense for the subjunctive in French. You use it when you wish something. In “Bette is hitting on Tex so that he will give her good grades.” Bette isn’t given good grades yet but she hopes she will.
If you want to say that she actually has good grades because she is hitting on Tex you should say something like:
Bette drague Tex, ainsi il lui donne de bonnes notes.
Using “ainsi” you show it is happening now and that there is causality link.
The question seems to be a good one, but on careful inspection of the English “present” subjunctive example, we see that it really means the same thing as the future example, and the phrase “so that he gives” is an idiomatic way of saying “so that he will give”.
In English, we often use the present tense to express a future event, even in the indicative mood; for example: Tomorrow I’m climbing Mount Everest.