From my point of view, they should practice more on returning fast on raising their hand up, the way to make them faster, gives more shuttle multi-drilling to them while ask them to raise their elbow to the highest point to generate the pace.
Furthermore, they should learn finger power and also some wrist power. This will increase their power of holding the racquet to generate the power with pace.
Last but not least, train more and get more experience by take part competition will help too. Hope it helps your students. Cheers.