"La escuela siempre es difícil."
Translation:School is always difficult.
It sounds really weird in english. I'm not a native speaker so I can't give you an exact answer as to why is wrong, but it sounds really really wrong.
"School always is difficult" is said in English. It has a slightly different inflection that "school is always difficult", but it is appropriate.
Here school is generalized, so the the is omitted in English. Sorry, it just sounds wrong to include it. Just as the la is required in Spanish. Both languages must be translated correctly.