"Deze tekst is belangrijk voor de klas."
Translation:This text is important for the class.
Nope, textbook = leerboek or studieboek if either of these is shortened, it can only be shortened to boek. Tekst really refers to the words/sentences themselves, the length can vary between e.g. a very short slogan, up to text of a whole book, or even the complete works of a writer, e.g.: "ik hou van de teksten van Mulisch".
adjectives lose their -e ending when connected to the described noun via forms of "to be" (zijn), so "de belangrijke tekst", but "de tekst is belangrijk". This is just like "de hond is groot" (the dog is big) but "de grote hond eet" (the large dog is eating). I think this is covered one of the earlier lessons on adjectives, if I'm not mistaken.