In this example :
A ( pupil ) I promise I'll bring my math homework tomorrow.
B ( teacher ) OK. But make sure you do.
I am just wondering whether make sure you will is more grammatically correct?"
Can you help?
No - will is not possible here. Remember the meanings of willthat we talked about in a previous thread - it can be a prediction or willingness. Neither or those meanings is being expressed here and therefore will is not relevant. The speaker wants "bringing the homework" to become a concrete fact. Therefore the first form of the verb (which expresses concrete facts - I live in Italy, Water boils at 100°C ) is therefore used.
make sure you do or you will? 
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