Note on a napkin: You can easily write a note on their napkin every day. Just a "Love Mom" is all that's necessary. This is the most simple way to encourage them and can be done often.
Encouragement of the week: Maybe one day a week (every Tuesday or another day they have weekly testing) you write them a note on an index card and put it in their backpack or lunchbox.
The big day: Give them a specific word of encouragement when they have a particularly big day like a mid-term or presentation. You can even do this on a little note card and put it in an envelope and put it in their backpack or lunchbox.
There are lots of things you can write on your encouragement cards. Here are a few ideas to get you started:
You studied hard and I'm proud of you
I love you and I'm proud to be your mom
Have a great day
I can't wait to do something fun this weekend
You make me so proud
Be confident. You are well prepared
I'm proud that you are working so hard
Look how far you have come
You are my shining star
All your hard work is paying off
Just do your best
Write your little notes on a label, napkin, colored index cards, note cards, and more. You can decorate it with some stickers, draw a smiley or heart, or even just leave it plain. You can do a few cards at a time so they are ready to add to their backpack or lunch box easily.