Google Voice will work for you and it’s free. If you get a paid service (Google voice also has a paid service) then you can create multiple numbers as needed. But given you are saying it’s a method to communicate with parents, Google voice would be a good option as you’ll likely only use one phone number per year and the means to communicate may end once students move up a year so one phone number that you can set a DND as needed and you can read texts online on your computer at voice.google.com or via the Google voice app on your personal phone. You set incoming calls to direct to your personal number without revealing your personal phone number or to the Google voice app or website. Google voice does not work with sending videos and quality of received videos is terrible, however images in and out are fine. Group chat does work on Google voice.
With WhatsApp, you reveal your personal phone number and everyone has to have the WhatsApp app. Those parents who don’t understand will likely call or text your personal phone number which defeats the purpose of using WhatsApp. You can use your computer or the app to use it. Great for sending voice audio, text, images, videos and documents, also voice video and group voice and video calls - could also be used accidentally too, something to keep in mind. Group chats are well organized. It is also free. You’d loose your privacy though, you could create a Google voice number and link your WhatsApp to that, I believe. Sometimes services won’t work with a VOIP number such as Google voice.
Texting might be a little too real time for a teacher though, you might want to consider something slower that won’t overwhelm yourself. Email or email lists is a good option too.
Or just use Google voice, set incoming calls to DND to to the Google voice app only, and log onto voice.google.com via your computer when you want to be “online” and when you log off you won’t hear anything. Google voice also sends you emails when you get texts or voicemails if desired.