"Ten un buen día en la escuela"
Buenos dias = Good day
Buenos días.
No es un buen día.
es un buen dia
That's not even a good sentence in English...
"School" in Spanish is "escuela".
You are good in school Eres bueno en la escuela You are a good student Eres un buen estudiante
I would say that this school is very good
Buenos días mi amor.
Estás teniendo un bueno dia ?
I am assuming you would like to know how to say "what a Day" in Spanish. That would be Que dia
To say 'I am going to school' in Spanish, you would say 'Voy a la escuela.'