Google Chat
Google chat is an additional app (available on your phone) that works with google classroom etc. In Google classroom you can push out notices etc. but in Google Chat you can live message whole classes or individual students. You can run this through the web, or as this video shows through a desktop download version.
Microsoft Teams Chat
Microsoft Teams has chat fully embedded in the new 365 experience. With the newest versions chat and Onenote are all together in Teams. Remember you can get Microsoft and Google for free as an educator and your students can also get the apps for free.
Remind
Remind is an amazing free communications app that I would highly recommend looking into. Although you can chat live with Google and Microsoft, Remind really gets into the nitty gritty of edcuation communication. For example in Remind you can add parents and you can turn off replies to conversations. I am seriously considering using this next year.
SeeSaw
SeeSaw is definitely more than just a messaging platform, for example you can also set tasks and students can create portfolios. However, the messaging service within SeeSaw is very good and is purpose built for education in a similar way to Remind. The video to the right is more instructional than promotional but I think covers the aspects nicely.
ParentHub
ParentHub is a slightly different type of messaging service, particularly in the free version (note the video describes some features which I think are only available for paid accounts). In ParentHub free you can create different channels (like a class or a year group) and broadcast a message to them. The feature that is very good is that you can then see who has read the message.
edmodo
Note that edmodo (also covered in the LMS section of this website) also has its own in-built messaging system. Through this you can message both students and parents. You cannot instant message a whole class as such, but of course you can post an announcement anyway to the same effect. It is a big advantage of edmodo as a LMS.
Slack
Slack brands itself as a communications service for Business. It does not mention education, however its features work perfectly well in the education context. I also wonder if you are working with older students it might be worth trying Slack to purposefully give you students more of a 'working world' feel.