Most likely, (and reports now suggest that it is the plan) the first solo film will be in 2019, so Lerman and O'brein will be 26 and 27 and both look their ages, so I'm not sure they could play a high schooler in 4 years time.
I'm sure it's going to be someone who is at least a teenager now, if not someone with a Garfield like baby face. *cough Sangster
I thought the newest suggested plan would be Civil War cameo, Sony produced solo before Infinity War, ensemble part in Infinity War, second solo after that.