I suppose either
(1) 数えられない = '...to be counted...'
or
(2) 教えられない = '..to be taught...'
@JimBreen
I've changed the sentence.
Thanks. I wonder if it was an OCR error.
If a native speaker adopts this, please consider linking one or both of the following.
[#11207992] That class is too big to be taught by one person. (CK)
[#11207991] That class is too big for one person to teach. (CK)