As the school systems are not dircetly comparable, I recommend not to use the word Abitur for referring to your graduation from an American high school. Note that although our system here in Austria is relatively close to the German one, we are using the word Matura instead of Abitur.
I would use something more verbose and precise, like talking about Highschoolabschluss.
Or, if you want to refer to the ceremonies involved you can say something like
Bei den Feierlichkeiten zu ihrem Highschoolabschluss hielt sie eine viel beachtete Rede.
As to graduation from college or university, the usual way to talk about that would be to mention the academic title you've earned:
Er absolvierte ein Bachelor-Studium in Harvard.
Sie machte ihren Doktor in Physik an der XYZ University in Exampleville.
Er hat einen Masterstudiengang an der Fachhochschule in Hagenberg abgeschlossen.
Note that throughout most of Europe traditional names for academic grades are slowly being replaced by the Bachelor/Master system introduced by the Bologna process, so things are currently changing in this area.