I went to the Japanese wWeekend sSchool today. I went to the Japanese Weekend School today.

Not sure what this means. If this is a specific type of school (as in a proper noun, note my above corrections.)

I hadid two tests that for investigated houw mush of amountany of last year's students understand and find the advantage.[not clear...] I did two tests that investigated how many of last year's students understand and [not clear...]

Perhaps you meant "find the difference"?

I had two tests that for investigate houw musch of amount of last year's students understand (the material?) and find the advantage. I had two tests that investigate how much last year's students understand (the material?) and find the advantage.

You have to give more context here. I'm having trouble correcting as I'm not sure what you're trying to say especially with the last four words.

I didn't have enought time to solveanswer every questions as well as I did last year. I didn't have enough time to answer every question as well as I did last year.

I didn't have enought time to solve every questions as well as similar to last year. I didn't have enough time to solve every question similar to last year.

After me and my siblings were done at the school, we headed (back) to our car. After me and my siblings were done at the school, we headed (back) to our car.

After me and my siblings were done the shoolfinished, we headed to our car. After me and my siblings finished, we headed to our car.

My brother noticed that a bird had dropped its dropping on our car. My brother noticed that a bird had dropped its dropping on our car.

My brother noticed that a bird dropped its dropping on our car. My brother noticed that a bird dropped its dropping on our car.

We had just had a car wash recently, so we were kind of disappointed. We had just had a car wash recently, so we were kind of disappointed.

"Upset" might be better.

We just had a car wash recently, so we were kind of disappointed. We just had a car wash recently, so we were kind of disappointed.

Disappointed may not be the right word here- look into definitions for sad or frustrated.

After we wengot back home, my dad washed the part of caffected part with water. After we got back home, my dad washed the affected part with water.

Or "cleaned the affected part" might be preferable.

After we went backreturned home, my dad washed theat part of car with water. After we returned home, my dad washed that part of car with water.

I want to get back my habit of writing a diary entry every day, because I was too busy towith my assignment this week and I could'n't write in every day. I want to get back my habit of writing a diary entry every day, because I was too busy with my assignment this week and I couldn't write every day.

"resume" might be preferable to "get back"

I want to get back into my habit of writing a diary entry every day, because I was busy towith my assignment this week and I could'n't write in every day. I want to get back into my habit of writing a diary entry every day, because I was busy with my assignment this week and I couldn't write every day.