You can say "Gakko wo hajimeruyo", but can't say "Gakko wa hajimeru yo." because hajimeru is a transitive verb. "Gakko wo hajimeruyo" means somebody starts/opens a school.
You can say Gakko wa hajimaruyo., but it means "The school will start, (but other things won't start.). For example, when someone asks you "Will the school and the store start again?" , you can answer "Gakko wa hajimaruyo." It implies the store won't .
The difference between "koto ga dekimasen \ koto wa dekimasen" is the same as the above example. If you said "koto wa dekimasen", it implies you can't do it, but you can do the other things or there are many things you can do. "Koto ga dekimasen" just means you can't do it. You don't mention aabout other things.
"Iza, ikou" means "Hey! Let's go!"
Iza as an interjection is used when you ask someone to do something with you. Iza has a different meaning like "at the last moment" or "in case of emergency" or something like that.
Kame wa umi no yoko ni magutta. It's odd.
Instead, "kame wa umi no yoko no shima ni ita" or " Kame wa umi wo yoko ni oyoida" may make sense.
I hope it will help you and my English makes sense.