The most important thing is the quality and robustness of
your coding. It must be clear and well-structured, not relying
on strange unexplained tricks. Meaning that if you continue
to program in the same style after graduation in the real world,
where programs may be 1,000 times larger and with correspondingly
more complex requirements, you should still be able to make
things word reliably.
 
So no giant functions that perform too many tasks. Have sensibly
chosen variable and function names. Make the structure of your
program visibly obvious. Work from a conscious design.

DIY, do it yourself. If you discover a function or class that does
a major part of any assignment for you, then obviously you may
not make use of it. There is a list of permissible #includes in
a separate document.

I grade programs by looking at them, running them is a secondary
concern.
 
For grades, good design is much more important than just happening
to produce the right answer. A good program that doesn't quite get
it right gets a better grade than a bad program with correct test output.
 
Inefficiency only has a negative effect on grades if it is really dramatic.
Reliability is much more important than speed, and generally the
programmer's time is more valuable than the computer's.