These are the years to learn the foundations of math.
1st-3rd
From 1st grade, students are learning about numbers, adding numbers, and fact families. By 2nd grade, they are practicing subtraction. 3rd is learning the multiplication tables. In all this time, students are learning to read graphs and charts. They are learning basic fractions and missing numbers.
4th-6th
By 4th grade, students are doing long division which requires estimation, multiplication, and subtraction. In 5th grade, percents are introduced where you need to understand fractions and decimals. By 6th grade, students are doing statistics and square roots.
7th-8th
In 7th grade, you start to get integers with positive and negative numbers. Algebra starts to get more demanding. By 8th grade, you must have mastered the basics of math including negative exponents and ratios/rates/proportions which you will need for Geometry, Chemistry, and Algebra.
What if a student is below level?
It is important to find out what knowledge is missing. A student can be behind a little or a lot.
The good news is that students can learn what they are missing and catch up with the class and be at level for the grade.
Take a look at the skills needed for each school year, and see if the student has it mastered. For example, students should not be using their fingers to figure out 3+4. Some adults use their fingers to count to do math, but never for 3+4. This means that the student has not had enough practice. When they were teaching addition facts in school, for any number of reasons, the student did not get enough practice to automatically know 4+5=9.
Well, it just takes practice. Even if the student is now in 3rd and they are doing times-tables, or 4th or 5th, if the student has to count to add under ten, they simply need to practice. They need to do worksheets, and computer exercises that repeat the numbers up to 10.
What if a student uses their fingers to count numbers up to 18, such as 7+9 or 13-6? It is at this point that some students stop practicing. Knowing the addition and subtraction up to 18 without having to use your fingers is one of the most useful skill a student will need for math from the beginning of 2nd grade.
Learning this is exactly the same as up to ten, but now it is up to 18. It just takes practice, practice, practice for a certain period of days or weeks without breaks. By making a decision to focus this skill for a student will change their future.