I was late to the home room today, because I was talking to the another teacher. I was late to homeroom today, because I was talking to another teacher.

You wouldn't usually say 'the' before a noun like 'homeroom' in English, you can also take out the space between home and room. In this context, it would be better to say 'another' or 'a different' teacher.

When I got into the class room, my classmates and the teacher wasere watching the video about Kramps festival again. When I got to the classroom, my classmates and the teacher were watching the video about Kramps festival again.

Change to 'to' instead of into, alternatively you could say 'when I went into'. Remove space between class and room. Use 'were' instead of 'was' because the subject is plural.

One guirl who came late, said "again?", and laughed. One girl who came late, said "again?", and laughed.

Spelling

It seems they like watching videos about Kramps festival video. It seems they like watching videos about Kramps festival.

I also like the festival, so I was happy to watch ithe video. I also like the festival, so I was happy to watch the video.

But it was more scary video thatn then video yesterday's. But it was more scary than the video yesterday.