<p>I mean, it is possible to <em>skip</em> them once in a while with few or no consequences ?</p>
<p>That entirely depends on the class.
Some classes don't care at all whether you attend discussion.
Other classes make participation mandatory and part of your grade.
In either case, the policy will be clearly stated on the syllabus.</p>
<p>Labs are entirely another matter... you can never just skip a lab.
Some science classes allow you to miss ONE lab during the semester, but you still have to make it up during the last week, when many classes have a lab make-up session.</p>
<p>I was a teaching assistant for Physics 7, and the policy then was that if you missed a lab without making it up, your grade for the whole class automatically dropped from A to B, or B to C, etc.</p>
<p>You can usually skip lecture whenever you want, but discussion is another story. I've had sections that let you miss one day, but I've also had sections that take off a whole letter grade for every class you miss.</p>