There is so much focus on teaching students the basics at the elementary level. Sadly, there are so many high school students who have missed some of the basics. Here are some of the common misunderstandings that I've seen high school students repeatedly struggle with, along with some intervention resources.
#1: Fractions...So many students who struggle with math are not able to visualize or conceptualize what it means that there are wholes and parts of a whole. Therefore, being able to use fractions--or decimals and percents--to manipulate and calculate is a struggle. Here are a few resources that I've seen used that do not appear babyish to high schoolers...
#2: Comprehending nonfiction....One of the skills that students struggle with is pulling the main idea and details out of nonfiction texts. Unlike fiction, struggling readers often do not realize that nonfiction texts use a variety of structures (spatial, compare & contrast, cause & effect, etc.). If they did, they would become more strategic readers and successful at comprehension. Here are a few resources I've used...