Strength of schedule is calculated from the opponents projected fantasy points allowed for the TE position. The best rating is +16 which gives the player the advantage against the defense and indicates an easy opponent, the worst rating is -16 which gives the defense the advantage and indicates a tough opponent.
Given all the uncertainty at tight end in Philadelphia right now, it would be unwise to dismiss any of the candidates to start out of hand. Yes, James Casey is the low man on the totem pole, and less likely to end up starting than either Brent Celek or Zach Ertz. However, the fifth-year H-back showed in 2012 while in Houston that he can produce when given the opportunity, and there’s a reason why Chip Kelly and company signed him.