You say you were "arrested" for attempted robbery, if you have not yet had a petition sustained against you (which is the juvenile equivalent of a conviction), then you don't need to worry about your record being sealed yet -- your lawyer needs to fight the case, reduce the charge, or get you on a deferred entry of judgment program where you won't need to worry about sealing the record. If you are now an adult trying to seal your record the law permits you to do so, UNLESS there is an allegation that during the commission of the attempted robbery you personally used a firearm or a weapon listed in PC 12020(a) of the penal code - in that case your are not eligible to have your record sealed.
HOWEVER, if you are not eligible under the law to have your record sealed, you may be able to withdraw your plea, petition the court for a motion to set aside and dismiss, or work something out with the DA and court. It really depends on the facts of your case. Bottom line: Talk to an attorney who regularly handles juvenile offenses and understands the process.
I can't say for sure without more information. You say that you were arrested. Were you convicted? What were you convicted of?
You are right that some crimes cannot be sealed from your juvenile record. However, without knowing more, I can't say for sure.
My advice is to visit with an experienced California expungement attorney. He or she will be able to look at your juvenile rap sheet and see what options are available.