Five sentences in English

I'm reading a book and when I find a word I don't know I create a sentence. Please correct my mistakes
1) These group dance classes are only for young adult's/teenagers, we don't accept children or adults who are older than 25 years old. ( the word is youth)

2) Such a ridiculous situation! The best students didn't do their homework. (the word is ridiculous) 

3) I like this film but the plotline is not like the book.(the word is plotline)
4) I'm grateful you came to help me in this difficult situation ( the word is grateful)
5) She is utterly lost when her parents argue. (the word is utterly)

Thank you!

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Blue is where I changed a sentence around to make it sound a bit smoother and green is where I fixed spelling errors.

In English 'youth' can be used to describe very young children as well as teenager's/young adult's, so the sentence didn't sound quite right as it was. If you'd like to use the word 'youth' with this sentence you could try something like:
"These are youth dance classes, we don't accept adult's over 25 year's old or children under 10"

Five sentences in English

I'm reading a book and when I find a word I don't know I create a sentence. Please correct my mistakes
1) These group dance classes are only for the youth/teenagers, we don't accept children or  adults people who are older  than 25. years old. ( the word is youth)
2) Such a ridiculous situation! The best students didn't do their homework. (the word is ridiculous)
3)I like this film but the plotline is not like in the book.(the word is plotline)
4) I'm grateful you came to help me in this difficult situation ( the word is grateful)
5) She is utterly lost when her parents argue. (the word is utterly)

Thank you!

Well done, I understand why you used the word youth, however, we don't always use it in that context.  Better would be children/teenagers/adults.

Five sentences in English

1) These group dance classes only for youth, we don't accept children and adult people who elder than 25 years old. ( the word is youth)

Youth is young people (adolescents) / young (in terms of age) (usage depends on the sentence) 

2) Such ridiculous situation! The best students didn't do their homework. (the word is ridiculous)

Ridiculous is amusing or absurd (ex: this comic book is ridiculous)

3)I like this film but the plotline is not like in the book.(the word is plotline)

Plotline is the course of what happens in a movie, book, or a play (ex: the plotline is so elaborate)

4) I'm grateful you came to help me in this difficult situation ( the word is grateful)

Grateful is the same as thankful. (She's greatful for the things that her parents have done for her)

5) She is utterly lost when her parents arrgue. (the word is utterly)

Utterly is the same as totally and completely (ex: I'm totally speechless & I'm utterly speechless)

Five sentences in English

I'm reading a book and when I find a word I don't know I criate create a sentence. Please correct my mistakes
1) These group dance classes are only for youth, we don't accept children and adult people who elder than 25 years old. ( the word is youth)  These group dance classes are only for youth. We don't accept children or adults older than 25. That is how you would write it, but it doesn't SOUND natural for an English speaker. Youth is better used in a sentence like "The youth of today are obsessed with their cell phones!" or "I am old and out of shape, but in my youth I could run very fast!"
2) Such ridiculous situation! The best students didn't do their homework. (the word is ridiculous) What a ridiculous situation. The best students didn't even do their homework!
3)I like this film, but the plotline is not like in the book.(the word is plotline)
4) I'm grateful that you came to help me out in this difficult situation ( the word is grateful)
5) She is utterly lost when her parents arrgue argue. (the word is utterly)

Thank you!

Great job! I hope you're enjoying your book!