Publishing URLs within an educational system and telling students you are publishing their links is one situation. Publishing links to assignments that are completed for a class to the outside world is a different situation. What I do not see is alerting students to the possibility that links to their blogs will be distributed outside their specific educational entity. They may not "care" or they may care very much.
Back in the old days, I would accept that other instructors might see any paper I wrote. If the instructor intended to publish it for other students to read, then I was always informed and implied consent by not objecting. But, if the instructor had started making copies and giving them to anyone they encountered all over the world, I would not have felt that was appropriate. Consenting to sharing within on sphere does not imply consent to a larger sphere.