Writing a Persuasive or Argumentative Thesis (in response to a prompt)

The single most important line in an essay is the thesis statement. It's a line found in an introduction, usually the last line thereof, which sets up the rest of the paper. When composing a thesis statement for a persuasive or argumentative essay, consider that it needs to do the following things:
Make a declarative statement of your position on the topic the rest of your paper will support.
Give the main idea of the paper and the three main points that you will use to support your claim
Outline the rest of the paper by presenting its three points in the same order they will appear in the body.
Note that your thesis should be written so clearly and independant of anything written before it, that it could be completely seperated from your paper and still make sense. Let's look at some examples of quality thesis statements.

Cell phones should be allowed in school because they can save lives, serve as learning tools and teach teenagers responsible use of technology.

Notice the independant clause which gives a clear declarative statement to explain the topic of the paper and the writer's position on that topic. This clause can appear at the start of the end of the thesis. Here's the same thesis with that clause put at the end instead.
Because they can save lives, serve as learning tools and teach teenagers responsible use of technology, cell phones should be allowed in school.
You should also notice that the rest of the thesis statement gives 3 points about the author's position that will be explained in the body of the essay. Without reading the prompt that this writer is responding to, you know exactly what the paper is about, the author's position, and what three things the author intends to talk about to convince us of the the position taken.
At no point in the thesis or introduction are these points elaborated on- that's what the body is there for. When you write clear statements that demand explanation, your reader is urged to seek out explanations in the rest of your paper. This thesis claims cell phones can save lives, so the reader will be looking for its author to support that claim with examples in the body. Because that was the first claim in the thesis, the reader should be able to find elaboration and support for that claim in the first body paragraph. Support and elaboration for the second thesis point should be found in the second body paragraph and so forth.

There are many ways to set up a basic thesis, depending on the writer's position. It's generally best to keep it as short and simple as possible. Here are some more examples.
Ex. A
High schools should adopt uniform polices because they decrease bullying, parental financial pressure, and school distractions .
Ex C
Becuase uniform policies restrict student comfort, expression, and general freedom high schools should not adopt them.
You'll notice that in the examples provided all three points directly supported the claim, and were written carefully enough so that each point was only one or two words. Example A is written so that all 3 points tell something that uniforms can decrease. Ex. C tells three things that such a policy restricts. This is called parallelism, or parallel structure, which is the repetition of the same grammatic device.
You may feel more nuanced on an issue and wish to explain 2 good things and 1 bad thing about a side of an issue, or 2 bad things and 1 good. While this isn't recommended, as it breaks parallelism, makes the paper more complicated and your position less clear, it is possible, and if done properly it can lead to balanced, quality writing. For example:
Ex D
High school's should adopt uniform policies because the benefits of reduced bullying and distractions are well worth the cost of one form of student expression.
A thesis written as seen in EX D can also set up a counterargument that acknowledges a good point from your opposition before explaining why it's not good enough to change your mind. We'll cover that more in depth in later videos. In the mean time, use the criteria and examples here to experiment with your own thesis statement.
Remember to always ask yourself if your thesis does the following three things:
Make a declarative statement of your position on the topic the rest of your paper will support.
Give the main idea of the paper and the three main points that you will use to support your claim
Outline the rest of the paper by presenting its three points in the same order they will appear in the body.