Welcome to the convoluted mess of commenting. There is a mix of technical issues at play and some hub design decisions.
First of all, all of the sites that provide the blue link for comments are Wordpress, which it actually appends to the post content in the RSS feed. If only other blog platforms did this.
Several of your students use tumblr; their platform does not even provide commenting. Tumblr users can enable disqus comments on their sites; see https://disqus.com/admin/tumblr/ However this will not provide the same same link in the aggregation feed. And I am not sure what Blogger does. I see someone using weebly too, I cannot say for sure if they have commenting.
So it gets messy when you provide the flexibility of letting students choose a platform (which I prefer doing). If you had them all use Wordpress.com, you'd have a uniform comment link.
You can change the settings on Feedwordpress so that comments are done on the hub site. The advantage would be all the commenting activity would be in one place; I am not 100% sure how these links work when the permalinks point back to the original site. The downsides are (a) that the comments are thus separate from the students work and (b) you have to moderate all of the comments, rather then them. Also the students would not get email notifications of comments.
I have done this in a course where we had all students use tumblr; but we also had the post links stay on the aggregated hubs site (see http://2014.projectcommunity.info/community/students/). This was because of the lead teachers desire to see all comments in one area.
Again, to me the students comments should be in the same place as their content, on their site. I imagine they struggle with knowing where to comment? Well, that's how the rest of the messy internet works (#lame excuse I know).
What I would do, and would be willing to see if I can try to make a theme modification for you, would be to adjust the template that generates the syndicated post output to work something like
is the post syndicated (there is a test); if so
is the platform anything but Wordpress
(this data is n feed info); if so
append to the end of the post Add a Comment (and link to the original post)
tumblr can be a problem if the students do not do the extra step to configure disqus.