My Chemistry and AP Chemistry classrooms are filled with students at differing ability levels, and I keep the expectations and rigor high, but that doesn't mean we have to lose the fun and the joy of learning.
Instead of worksheets, try activities and games. These can be as complicated or as simple as you want -- many people assume games are "fluff," but the right game can be incredibly challenging. Students learn more when they're invested, and they're invested when they're enjoying the learning.
Follow this blog for science teaching tips, hints for scaffolding difficult concepts, and ways to keep your expectations and rigor high, while making your classroom a safe place for students to make mistakes and learn.