Many students try to tackle sightreading all at once and they can get frustrated if the music seems hard. I am teaching my students to first look at the rhythm and practice only the rhythm. After that, students practice the notated pitches with the rhythms. The next step is to add the bowing and after that, reading the music as printed. I believe students will experience more success if sight-reading is broken down into these smaller steps.
This is an example of a sight-reading exercise that follows this process:
You can download this assessment for violin, viola, cello and bass HERE: