Weird. We were having a problem where lots of our skill pages were getting our

<noscript>text used as page descriptions on Google SERPS.  We added these comments, and Googlebot reverted to using our meta description as the page descriptions in SERPs.  It could have been a freak coincidence that Google stopped using our <noscript> text right after we implemented the tags, or possibly Google was (possibly accidentally) supporting them for web search awhile back when we originally did this, and now has stopped supporting it.  Anyways, our SERPS remain clean of our <noscript> text today (<a href="https://www.google.com/search?q=site:www.ixl.com/math/grade-5" target="_blank">example</a>).</p> <p>John Mueller recently commented on that Quora thread saying it won't do anything for web search, so IMO that puts this to rest.</p></noscript>

It sounds like Martijn solved your problem, but I still wanted to add that robots.txt exclusions keep search bots from reading pages that are disallowed, but it does not stop those pages from being returned in search results.  When those pages do appear, a lot of times they'll have a page description along the lines of "A description of this page is not available due to this sites robots.txt".

If you want to ensure that pages are kept out of search engines results, you have to use the noindex meta tag on each page.