The sSad tThing

Title capitalization rules.

A more natural title might be "Some Sad News"

I'm so disappointed to hear that I will leave themy class.

"The" class requires that you have already mentioned this class in your text; you need to first introduce it to your readers

I'm studying now a week laterfor next week.

I didn't understand what you meant by this line. I wrote something that would make sense given your context.

But I verI really like my (head) teacher.

Try to avoid beginning sentences with "but."
"Head teacher" usually isn't needed, unless you need to distinguish this teacher from other teachers according to position

She is very consideratione and humorous.

She isn't like some other (head) teachers who always chide students for some small reason.

And she usually thinkings from the standpoint ofor students.

How good she is inShe is so great for our school.

More natural phrasing

The sSad tThing

Most words in English titles are capitalized, except for small connector words like "the" "of" "and" etc. The first word is always capitalized, so "The" is capitalized even though it's a small connector word.

I'm so disappointed to hear that I willhave to leave themy class in a week.

Since you're sad, my correction assumes that it's not your choice, that you have to leave due to some unspecified reason. If there is a reason and it's not private, it would be clearer to include it in the sentence.

I'm also not sure if "class" is the right word here. That usually describes one room's worth of students. Are you graduating from one level and moving up to the next? Are you leaving the school?

I studying now a week later.

I think this needs to be combined with the previous sentence. It doesn't make sense by itself.

But I veryI like my head teacher a lot.

"But" starts a dependent clause, and sentences need to have an independent clause to be complete. "But" means that whatever you say next goes against the previous thing in some way, and that doesn't make sense when there's no previous thing.

She is very consideratione and humorousfunny.

"Humorous" is okay, but "funny" sounds more natural when you're talking about a person. We're more likely to describe situations or written texts as humorous.

She isn't like some other head teachers who always chide students for some small reason.

I think "chide students for some small reason" is fine. If you want to sound more conversational, you could say "nitpicks students" or "nitpicks students' mistakes." The word "nitpick" conveys both the verb (criticizing someone) and the noun (small reason, unimportant thing).

And she usually thinkShe often thinks about things from standpoint for studentsa student's perspective.

You can also say "she's good at putting herself in students' shoes."

How good she is inShe is an asset to our school.

Your phrasing isn't wrong, but it sounds formal and poetic to me. This kind of statement would usually be short and direct in English, in our standard subject-verb-object word order.