A lot of these are appropriate for middle school as well as high school - Share a few fun challenge problems with your students (integers, geometry, and more!)
2. Algebra Fact - @AlgebraFact - for higher level math facts
Postings include Abstract Algebra, Number Theory, and higher level Algebra. These would be interesting for advanced students or higher grade levels.
3. Math Giraffe (of course!) - @TheMathGiraffe - for the #WeeklyMathematician series plus teaching articles, math education links, and more
New Mathematicians are posted each Friday. Just put the fact on the board for the week, and then download the free crosswords to review at the end of each semester -- an easy dose of math history for your class! Read more about how your class can join us for the Weekly Mathematician here.
4. Geometry Dad - @GeometryDad - for really cool animated images to inspire Geometry appreciation
Any grade level will enjoy these neat GIFs that Geometry Dad puts together for his kids. They show the beauty of Geometry in a fun way!
5. NCTM Illuminations - @NCTMIllum - for lesson plan links
These games, investigations, and lesson ideas are so helpful! I love all the content on the Illuminations website, but can never browse through it all. Through the tweets, I find myself slowly being exposed to fun new ideas over time. The lessons are wonderful for inquiry / discovery learning.
6. On This Day in Math - @OnThisDayinMath - for daily math history facts
These mathematician bios and math history facts (often more than one per day!) are great for students or classes that are interested in what happened on today's date to impact mathematics.
7. Ben Orlin - @benorlin - for humor and funny drawings
There may not be as much to share with younger students, but older ones may appreciate the humor, and at least you'll get a grin as you browse through. I just love these fun math cartoons.