It really depends on your postsecondary destinations. Are you aiming to apply in the sciences area, or the arts? If English is difficult for you, I would do that in summer school. It is used for all application admissions, and you can repeat it if necessary. Actually, English is usually the easiest to take in summer school, unless you really, really struggle with another course and wish to get it out of the way.

Almost every single person I know who took a full course in summer school took English. They didn't enjoy English so they wanted it out of the way, and there wasn't any specific information that they needed to retain for their programs of choice. If you take a science in summer school and plan on applying for something in that area, it will be harder to retain the knowledge, and so your courses will be harder.