Sure! Here are some example sentences using comparative adjectives and adverbs in English for sixth-grade students:

My brother is taller than me. (形容词比较级)

She studies harder than anyone else in our class. (副词比较级)

This apple is sweeter than the one you gave me. (形容词比较级)

They run faster than the other team. (副词比较级)

My new house is bigger than the old one. (形容词比较级)

He speaks English more fluently than most of his classmates. (副词比较级)

This book is more interesting than the one I read last week. (形容词比较级)

She dances more gracefully than anyone I've ever seen. (副词比较级)

Remember, when using comparative adjectives or adverbs, we usually use "than" to compare two things or people. Also, most adjectives and adverbs form their comparative by adding "-er" to the end.