Wikipedia:WikiProject Classroom coordination exists to provide guidance to educators who incorporate Wikipedia writing assignments into their classes. Post questions for experienced Wikipedia volunteers at Wikipedia talk:WikiProject Classroom coordination. Wikipedia:school and university projects - instructions for teachers and lecturers and Wikipedia:School and university projects - instructions for students are useful resources. There is also a syllabus boilerplate that you may want to use.
Try having students start a requested article or expand an existing one:
Wikipedia:Most wanted stubs
Wikipedia:Most wanted articles
Wikipedia:Requests for expansion
Tell students to take existing orphaned articles and link them into appropriate places:
Tell students to fix spelling, factual, grammatical, and other errors.
Wikipedia:Pages needing attention
Wikipedia:Requests for feedback
Tell students to add wikitext markup, links, and standard sections to poorly-edited articles (i.e., to wikify articles):
Have students translate articles into English from another language.
Wikipedia:Translation into English.
Students could translate our featured articles into other languages or write their own.
Complete list of language Wikipedias available
Have students contribute to a subject-matter area that has generally been neglected.
Wikipedia:WikiProject Countering systemic bias
Students could work on the collaboration of the week.
Students could help with the projects at one of the Wikiportals (pages organizing projects on a broad subject area)
Students could add citations to existing pages, thus helping to improve the credibility of Wikipedia while they learn the significance of citing sources. (Wikipedia:Citing sources and Wikipedia:Forum for Encyclopedic Standards.)
Fork selected problem articles into a local Wiki for a class so students can edit them collaboratively. The resulting revisions can then replace or be incorporated into the original Wikipedia articles.
Students can participate as help desk volunteers, developing skill by answering questions from other Wikipedia users, and of course ask questions of their own:
Note: in many cases, answering Help desk questions amounts to looking up the relevant Wikipedia policy or manual article. Students can learn much about how Wikipedia works by studying questions and answers on the Help desk, and learning how to look up the answers. In fact, teaching students how to answer Help desk questions would be a good way to teach them to be Wikipedians.
For many courses of study, related WikiProjects exist. Students may obtain guidance from Wikipedians with similar interests, and find lists of open tasks in one or more WikiProjects corresponding to their majors. Just a few examples:
Wikipedia:WikiProject Free Software