The grammar is fine, if in each case you're referring to a specific group of students. If you mean that the book is useful for ANY student whose first language is English, then you should just say "students," not "the students." The same is true in the second sentence: if you mean that in general, students who have low grades should participate more in class, then you do not need the word "the." You should only say "the students" and "the class" if you're talking about a few specific students in one specific class.

Hi Hamid I'm from Iran too. Do you want to practice English together? For speaking skills