This app was mentioned in 9 comments, with an average of 2.56 upvotes
Another suggestion along the lines of Duolingo (if you have an Android device) is Semper. It gives you a word each time you unlock your phone or open an app.
Not sure if you can do sentences on it, you probably can but what I used to use it Semper and it worked pretty well for me. I stopped using it when I switched phones to a pixel and unlocking became so quick and I started feeling comfortable not having it anyway, but I did use it for probably a year or so and can recommend it. They have some Japanese decks that are premade, but I'm sure you can make your own cards if you want to too.
https://play.google.com/store/apps/details?id=co.unlockyourbrain&hl=en
you should check this one. it has custom packs added by users, but its super easy to download it and check quality (rating system)
Semper is great for learning a new language.
Does the school you teach at offer Japanese? You could ask the Japanese teacher for advice. As for just getting bits of studying in during the day I would suggest Semper and Memrise. I have those both on my phone, Semper gives you a little puzzle every time your lock screen comes up, and memrise you can do in 5-15 minute chunks when you have time. You can also listen to Podcasts on your commute. I really like JPod101 and Pimsluer. You won't become fluent or learn a lot of grammar by just doing those things, but you can learn a lot vocabulary and set phrases.