I encourage you to allow your students to OWN and take OWNERSHIP of
their sketch journals. Let them get trained/used to the idea so that
when they get to 7th or 8th grade keeping a sketch journal will come
naturally for them. Training them also includes letting them know when
to put it down. Sketch diaries are always (with their scribbling,
doodling, etc) a vital tool for knowing how your art student feels
about so many things. Collect them (even assign someone to collect
'em) if you want, but return them when you're done with your lecture.