Hello aspirant...
Yes you can appear for the mains exam in 2020 through the open school marks .There is no age limit for JEE mains.
Age Limit to appear JEE Main

There is no age limit for the candidates aspiring to appear JEE (Main) examination

The candidates who have passed XII examination in 2017, 2018 or appearing in 2019 irrespective of their age can appear in JEE (Main) Examination  2019.

The candidates should satisfy themselves about the eligibility of age limit requirements of the Institutes in which he/she is desirous of taking admission.

Year of Appearance in Qualifying Examination

Only those candidates who have passed their Class 12th Examination or any equivalent qualifying Examination in 2017 or 2018; or those who are appearing in their Class 12th Examination or any equivalent qualifying examination in 2019 are eligible to appear in both January 2019 and April 2019 JEE (Main)-2019 Examination.

Candidates who passed Class 12th/Qualifying examination in 2016 or before as well as those who will appear in such examination in 2020 or later are not eligible to appear in JEE (Main) 2019.
Subjects in the Qualifying Examination

Candidates must have taken at least five subjects in class 12th/qualifying examination with Physics and Maths as their core subjects in order to be eligible for writing JEE (Main)-2019. The candidates who have taken four subjects are not permitted to write JEE (Main)-2019 and if such candidates appear in JEE (Main)-2019 by furnishing wrong information, their rank/result will not be declared.

Hope it helps you.
Best of luck.

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