Can You Use "It Is Because" to Explain Your Opinion

Technically, there's nothing wrong with the construction "It is because..." itself. But in your example, it's not terribly natural. As you suspect, "This is because..." is much more natural as in (1):

(1) I agree that school uniforms are good. This is because they are easy to wear.

Better yet, even delete 'This is' and make them one sentence as in (1'):

(1') I agree that school uniforms are good, because they are easy to wear.

That said, there are times when you'd use "it" instead of "this" as in (2) and (3):

(2) It is because school uniforms are easy to wear that I agree that they are good.

(3) When|If I agree that school uniforms are good, it is because they are easy to wear.

Note that (2) is an it-cleft construction, and that "it" is referring to something in the same sentence in both (2) and (3).

There are other times when you'd use either "it" or "this" as in (4):

(4) A: Why do you agree that school uniforms are good? B: This|It is because they are easy to wear.

Here, you can delete "This|It is" as in (4'):

(4') A: Why do you agree that school uniforms are good? B: Because they are easy to wear.