Do NOT call this series "Onegai Sensei". If it were called "Onegai Sensei" it would be titled "おねがい☆先生". But it isn't, it is titled "おねがい☆ティーチャー", note the katakana reads "tei-cha". The katakana is derived from the English word 'teacher', thus it is known as "Onegai Teacher". Those who do call it "Onegai Sensei" have literally took the English word 'teacher', and then translated back into Japanese, and romanized it once again into 'sensei', which if you think about it, is just their way to show off that they know a little Japanese.