"Decent" regards quality. "Serious" regards behavior. A serious student is someone who puts in a lot of effort and doesn't joke around or skip school, but that doesn't necessarily mean that the student is getting good grades. Likewise, a decent student is someone who gets good grades, but they could also be sleeping in class or playing video games. As long as the decent student is good at learning, their decency remains, and as long as a serious student is humorless and focused, their seriousness also remains. Example: "Billy is a decent student, but he's always reading comic books. He could be so much better if he applied himself!" "Gloria is a very serious student. I've never seen her chat in class, and she always has all of her homework done perfectly." Note that a decent student can also be a serious one (and vice versa). Behavior does not define a decent student, and grades do not define a serious one. _______________________________________________ Alternatively, you could use a lesser definition of "decent" to mean morally or ethically decent, in which case a decent student is someone who behaves themselves very well, and they aren't defined by either their grades OR how seriously they take their work. Example: "All of my kids are really rude, except for Walter. He's a pretty decent student; he always brings me apples and thanks me for my hard work." However, since people rarely use this definition (especially not in this context), I'd say to typically interpret that word with the first definition I gave.

He's a decent student. = he is a good student - not the best and not the worst. He's a serious student. = he is serious about studying - dedicated to studying.

Decent is used the same as 'good'. A decent student would be one that doesn't cause trouble, isn't rude, etc. A serious student, however, would be one that tries really hard and studies a lot, shows a great interest in the subject, etc. If you take something serious, it means you find it important.