By exploiting the old man glitch in Generation I, a Pokémon can be acquired at a level higher than 100. Also in Generation I, any Pokémon can also be raised to a level above 100 via the Pokémon merge glitch; however, Pokémon in the Slow experience group need to be merged with a glitch Pokémon who requires even more experience at level 100. These Pokémon can continue to be leveled up with Rare Candies until level 255. Whenever a Pokémon over level 100 gains any amount of experience, its level will revert to 100, and if a Rare Candy is fed to a level 255 Pokémon, it will revert to level 0.
So, yes, a Pokemon can exceed level 100 in gen I and the highest level a Pokemon could be is 255.