Introduce the concept of integers. Draw a line on the board where all the students can see it. Write "0" at the center of the line. Then add the positive numbers to the right of the zero, and the negative numbers to the left of the zero. As you are writing the numbers, explain to the students what you are doing. When you are finished, ask the students for examples of things that can be measured with negative numbers.
Use a bank account as an example of using both positive and negative numbers. Although seventh graders may not have their own bank accounts yet, they are old enough to understand the idea. Explain to them how someone can have a negative amount of money if they overdraw the account. Using money as an example with provide students with a clear picture of how something can be counted with a negative number.
Provide each student with paper money. This will be used to role play a series of interactions that will demonstrate to students how to add and subtract integers.
Ask for three volunteers to come to the front of the room with their money. Tell the rest of the class to keep track of the amount of money each student has. Then provide the volunteers with instructions on how to exchange their money with each other. For example, you can tell student A to "buy" a pencil from student B. Then have student B offer to sell student A a notebook that will cost more money than student A has. Student A will then ask student C if he can borrow money to make up the difference. At the end of the interaction, ask the rest of the class how much money each student volunteer has. You can create an endless series of similar scenarios to allow all the students an opportunity to practice adding and subtracting integers.