French greetings have subtleties between the "tu" and "vous" form. Using the "vous" is a mark of respect. Generally, you should NOT use the "tu" form with a teacher, a boss, or someone that you do not know personally. "Tu" is reserved for your friends and acquaintances. IF you do not know what to do, start with the "vous" then the person will invite you to use the "tu" form. : "On peut se tutoyer".
Also, using just "Excuse me" is not completely proper: By saying just "Excuse me!" you are giving the other person the *order* of excusing you. In very formal or proper situations, it is recommended to use "Je vous prie de bien vouloir m'excuser" Trans: I am begging you to be willing to excuse me" It's long, very polite, and will show that you have mastered French greetings. If you can do more, you can do less. ;)