You are not the school. However, I see no legal liability in your adding a gold seal provided it is a generic seal not something like a school logo or embossed with any school motto or official indicia of any kind. The problem is printing certificates without permission. You violate both copyright and trademark law when you do this. Likely the school would provide you a permission letter if you ask, as the school is surely doing online certificates just to save money and pass the cost of printing on to the students. There is some chance of an implied license here is the school chooses to force students to do the printing and authorizes students to print out certificates.
The agreement you need is not from the student, it is from the school. The student is not going to sue you for infringement, but the school might. However, I think since the school has a vested interest in having its certificates look good, I think it's actually quite likely they would approve of your doing this.
Look for the win-win opportunity in any problem like this. Why not approach the school and pay a small royalty to the school in return for being an "official printer"? The school is obviously cutting certificate printing costs to save money, so it seems quite likely they would be eager to turn the certificates into a money maker through licensing some printer like you to be their "official printer". There's where your real opportunity lies. Good Luck, and, you're welcome.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.
Checking a box with the agreed statement is sufficient to create a contract. But that is the least of your problems here. You cannot legally add this gold sticker to the certificates without approval from the on-line school that issues the certificates. In so doing, you are creating a derivative work in violation of copyright law (unless you receive permission). Further, you may also be violating trademark law by associating the trademark of the institution with a form of certificate that it did not approve. Without approval from the on-line school that issues these certificates, you should not be printing copies of the certificates in which you add a generic gold sticker. That can land you in hot water (i.e., an expensive law suit).
I am not your lawyer and this is not intended to be legal advice on which you rely. My answer is merely intended to assist you in understanding some of the issues that you face so that you can make an intelligent choice when you hire legal counsel.
Checking off a box online is sufficient to create a binding contract, however you have additional problems with potentially infringing the intellectual property rights of the organization which issues the certificates. I would recommend that you hire an attorney and work directly with this organization to avoid liability and protect yourself.
This response to your inquiry is for general purposes only and not for the purpose of providing legal advice or establishment of an attorney-client relationship.