How can I re-enter the US with my EB-5 Green Card?

You need legal advice about this situation from your immigration attorney. There are involved and nuanced issues present here.

Since you plan to return to the U.S. in March 2027, your absence will be less than one year, so you should generally be able to re-enter using your valid Green Card without needing a re-entry permit. To avoid any questions about abandoning your permanent resident status, it is advisable to keep evidence that your stay abroad was temporary, such as your son's high school enrollment and your plans to permanently relocate to the U.S. in March 2027. As always, please consult with your EB-5 immigration lawyer.

A green card is a permit to reside in the US, which can be lost or taken away if CBP thinks you have abandoned your intention of permanently living in the USA. Any lengthy absence from the US, particularly more than 180 days, could result in CBP sending you to secondary inspection when you go through immigration upon return. In secondary inspection, they will likely ask questions such as: why have you been away so long; what were you doing out of the US; do you have a job outside the US? Do you have a home outside the US?
There is a saying, "home is where the heart is," which is a sentimental way of asking where " home " is to you. If the US is your true ‘home," then you should be able to prove that you have a home here, your things are here, your pets (if any) are here, your bank accounts are here, and you file your taxes here, etc. There are many justifications for being outside the US, including the fact that your child is finishing school in your country of origin. If you return and properly refer to this return as “coming home to the US’ and don't say your are visiting the US, and you have school records for your child and can show that you were only out of the country because your child needed to finish his or her schooling, and you needed to be close, that will typically be accepted, so long as you have obeyed the rules relative to filing tax returns and can demonstrate that you have a home or abode here in the US.
Absences of greater than 365 days are riskier, and we recommend that you obtain a re-entry permit if you will be out of the country for 365+ days. This request must be filed prior to your departure, will often require that you wait until your biometrics are completed, and then, once issued, allows you to remain outside the US for up to 2 years without being considered to have abandoned your residence.

It's impossible to see the future. As long as you are back within a year, you have not been 'deemed to abandon,’ but any stay outside the US of over 6 months does change the calculus. There are a bunch of things I would consider, including a reentry permit, a return sooner, as well as certain US-based arrangements, including accounts, property, etc. The climate is just not as forgiving as it has been in the past.