Write a topic sentence. Topic sentences convey to the reader what a given paragraph will seek to demonstrate. Topic sentences for grammar school may be short, but they should clearly lay out what the paragraph will discuss. These sentences should focus on one clear idea only.
Add supporting information. The next two or three sentences--an appropriate number of sentences for a grammar school paragraph--should build upon the topic laid out in the topic sentence. Supporting information can include facts, claims, other details and even questions that elaborate the main point; whatever the supporting information, each sentence should be related to the topic sentence in some way.
Finish with a concluding sentence. The concluding sentence is like a bookend, a partner to the topic sentence. The concluding sentence is the reverse of the topic sentence; while the latter states what will be discussed in the following paragraph, the former states what has been discussed.