A thesis statement is simply the main idea you want focus on in your essay, along with the points you will develop to support or explain your main idea. In other words, a thesis statement is like an outline of the essay, acting as a preview for the reader.
Your assigned topic is a sort of two-part thesis right now, the first part being that homeless students have attention span difficulties and the second being that teachers can do something to help them. So, you have the beginnings of a thesis statement, and what you will need to do is decide what kinds of points will support these. Your research should suggest to you the points that you will want to develop.
A few possibilities come to mind. I have never done any formal research in this area, but I have had homeless students. Students who are homeless often have short attention spans because they are hungry, exhausted, and disorganized. So, these are points that can be developed for the first part of your thesis.
What teachers can do to help students overcome their difficulties, the second part of your thesis, is try to tailor their teaching in a way that addresses each of these difficulties, in other words, to create accommodations for each student. Again, I have only my own experiences to draw upon as I think about this problem, which is truly a tragedy.
Putting all of this together, here is a thesis statement I might have if I were assigned to this topic:
Homeless students suffer from shorter attention spans because they are often hungry, tired, and disorganized, and it is up to teachers to address each of these problems through accommodations, such as arranging for students to eat breakfast at school, helping students schedule some "down" time during the school day, for example, sleeping in study hall, and providing space in the classroom for students to store their books and papers.
Of course, ultimately, the problem of student homelessness is a social problem that the teacher alone does not have the resources to attack, so any teacher with homeless students should be trying to line up resources to help the family solve the basic problem of homelessness.
The first answer covers everything you need to know to write a thesis or paper. The reason I am adding to this answer is that homeless students in the shelter where I volunteer has students ages 16-24. Homeless students are often hungry as they tend to have no money to buy food or their food was stolen while they slept. They are often tired because they slept outside in the cold, sometimes in below zero weather, have shoes which hurt their feet because they are too small, and they walk too far to school because the city bus doesn't go that direction. They have no place or time to study, and quiet doesn't exist. Shelters are crowded and noisy places where you have to be careful who you trust as someone may take the backpack with your homework in because it looks like a bag with either food or clothes in it. Homeless students have it so much harder than people know; I am amazed how hard they try to stay in school.
Is this a good thesis/hook introduction for a classical argument?
Homelessness is the leading problem that every continent is facing. The government are ignoring this problem. It has affected many families especially the young children who are growing up and are going to school. People who are homeless should not be ignored or judged by their life structure, but instead others can assist them to do better. The government can also help build homeless shelters and provide for their needs. How can the United States say they want freedom and rights for all their citizens and refused to give rights to the homeless? It is time to make a change and the change need to happen now.