Mega Sceptile is a pretty damn shaky check to all of those Pokemon and can't safely switch in on any of them. Even in its singular niche of being a fast check to most offense, it's got competition with Mega Alakazam and Mega Beedrill and can't break a lot of the typical defensive cores seen on other archetypes either. C would be generous for it.
The definition of a check, by an article of Smogon:
Pokémon A
checks Pokémon B if, when Pokémon A is given a
free switch into Pokémon B, Pokémon A can win every time, even under the worst case scenario, without factoring in hax.
Taking that scenario:
252 SpA Sceptile-Mega Leaf Storm vs. 0 HP / 0 SpD Greninja: 612-720 (214.7 - 252.6%) -- guaranteed OHKO
80 Atk Sceptile-Mega Earthquake vs. 0 HP / 0 Def Greninja: 234-276 (82.1 - 96.8%) -- 62.5% chance to OHKO after Stealth Rock (Poison type Greninja)
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252 SpA Sceptile-Mega Leaf Storm vs. 0 HP / 0 SpD Tapu Koko: 292-345 (103.9 - 122.7%) -- guaranteed OHKO
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252 SpA Sceptile-Mega Leaf Storm vs. 248 HP / 16+ SpD Tapu Fini: 330-390 (96.2 - 113.7%) -- guaranteed OHKO after Stealth Rock
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252 SpA Sceptile-Mega Hidden Power Fire vs. 0 HP / 0 SpD Tapu Bulu: 148-176 (52.6 - 62.6%) -- 99.6% chance to 2HKO after Grassy Terrain recovery
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252 SpA Sceptile-Mega Leaf Storm vs. 252 HP / 24 SpD Landorus-Therian: 270-318 (70.6 - 83.2%) -- guaranteed 2HKO
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80 Atk Sceptile-Mega Earthquake vs. 252 HP / 0 Def Heatran: 320-380 (82.9 - 98.4%) -- 68.8% chance to OHKO after Stealth Rock
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252 SpA Sceptile-Mega Dragon Pulse vs. 0 HP / 4 SpD Zygarde: 314-372 (87.9 - 104.2%) -- 62.5% chance to OHKO after Stealth Rock
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252 SpA Sceptile-Mega Leaf Storm vs. 4 HP / 0 SpD Tyranitar in Sand: 306-362 (89.4 - 105.8%) -- guaranteed OHKO after Stealth Rock
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252 SpA Sceptile-Mega Dragon Pulse vs. 0 HP / 4 SpD Latios: 278-330 (92.9 - 110.3%) -- guaranteed OHKO after Stealth Rock
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And so on...
I never said that Mega Sceptile will be a good switch-in for none of those mons I talked about (although it can come in Water Shuriken from Ash-Ninja all the time, Horn Leech from Bulu...), I said it can defeat those mons, what sets it up as a check for them. It won't be top-tier, for sure, but C is too low for it.