Why is it extremely difficult for me to concentrate, and why do I yawn repeatedly?
It could be many factors: lack of interest, screen time, poor sleep, inconsistent sleep, lack of hydration, too much sugar in your diet (causes crashes), eating before class, not eating enough before class, poor habit formation early in life, lack of physical exercise. There are many, many factors. However, to begin with, you should examine a few things in your lifestyle:

1) What is your diet like? when do you eat? do you eat consistently the same times each day? do you eat a carb heavy diet? 

2) Do you get physical activity? Are you hydrated or always drinking diuretics? 

3) Examine your sleep schedule. Inconsistent sleep can be worse for you than less sleep. 

4) Do you care to be in class? Are you treating your classroom like a job? If not, you should be. 

5) Journal your day so you have a record. After two weeks, identify poor patterns in your lifestyle.

6) After you take a deep look into your lifestyle, and the consistency of your lifestyle, consider speaking with a profesional about trying ADHD medication and integrating those medications holistically into your lifestyle. 

Nobody here will know why you are the way you are, but those are some key areas of life that you can personally make changes to.