The wording is 100% accurate, although a bit antiquidated. (It seems to me the question was written in the late '60s or early '70s.)
In elementary schools, teachers are often not required to have anything but a degree in elementary education. What that means is that they need not study math, English, science, nor anything else --only teaching. This does not excuse them from being able to complete simple math problems though.
My whole life I’ve had really good teachers, but I’ve known of a few really bad ones. Your child’s teacher does not appear to be one of the good ones. Part of math (and something your child should in fact be learning) is how to translate between words and math. When your child hits college he’ll need it. If his teacher cannot perform at a second grade level then he should not be teaching.
Complain to the principal and the PTA. Show them the paper. If necessary, make copies. Circle the top two shells and label them “Friday”. Circle the bottom four which your son correctly drew-in and label them “Saturday”. Label the word “shells” as “subject/antecedent” and draw an arrow from the word “all” to the word “shells”. I’d use a red pen. Ask why a full grown adult can’t do the problem that all the children did correctly.
Don’t take it lying down. I hate arrogant, snotty, ignorant, listless, bullying teaching. Good teaching comes only from an interest in the children and and interest in the material being taught. Were that interest there these kinds of mistakes couldn’t happen. Even if the teacher was just having a bad day, as already noted, the fact that all the students got this wrong should have sent a red flag up for him to re-check the problem.
It’s perfectly OK to begin a sentence with a conjunction, as we do it all the time in speach. It is frowned upon for children learning language because they are still learning correct speach patterns. The use of the word “and” (or “but” or whatever) signals a complex thought --which is (at 7 years old) still a bit difficult to construct outside of a single sentence. As they move into 6th or 7th grade their teachers will allow them to begin using “and” at the beginning of sentences because their thoughts are becoming more cohesively complex. In otherwords, they will be able to begin constructing thoughts that require more than one sentence to express.