Please check my writing. Thank you!

Dear Sir/Madam
I am writing to express my dissatisfaction with the English course I am currently attending at your language school. It does not live up to my expectations at all.
To begin with; in your brochure is stated that there would be no more than 10 students in one class. However, there are 14 students in my class. To make matters worse, the proficiency level of my class mates varies considerably. As if that had not been enough, the school facilities are in neglected condition Yesterday I wanted to sit on a chair and it broke down. If it had not been for my friend, who rescued me, I could have broken my neck. When I complained to the office staff, they started to laugh and told me that the chair had broken down because of my weight. I have never felt so humiliated in my life.
The most serious complaint, however, is the quality of your teaching staff. My teacher is always unpunctual. Today he came one hour late and did not even apologise. In addition, he is very impatient with his students and makes fun of them when someone makes a mistake.
To sum up, the low quality of your language school does not correspond to the standard you had promised in your advertisement. I ask you to take immediate action, otherwise I am going to write to the national language school agency. I hope that this matter will be dealt with swiftly and appropriately.
I am looking forward to hearing from you soon.
Yours faithfully
Very good, just a couple of little points

classmates - one word

As if that were not enough

in a neglected condition

and it broke. the chair had broken because. 'Break/broke down' is not used for when objects simply break. It refers to a mechanical problem - a car breaks down, not a chair.

always unpunctual - does not sound natural. 'always late' is much better. The more complicated word is not always the best option.
You've been very helpful. Thank you!