Words such as ‘similar/different and problem/solution' students organize important information in textbooks.

An Antonym is a word opposite in meaning to another word, and this definition applies mainly adjectives although some other words such as: adverbs, verbs, and even abstract nouns also have antonyms.

Antonyms enrich our vocabulary providing multiple perspectives when communicating ideas.

Now, a synonym for the word Antonym is "contrasting" or "contrastive"  

Thereby, the above statement can be completed like this:  

"Contrasting/Contrastive words such as ‘similar/different and problem/solution' help students organize important information in textbooks."