Sql-server – Microsoft SQL Server – Permission to create a Snapshot

There is no need to grant CONTROL on the schema.
The permission required to DROP SCHEMA is either CONTROL on the schema or ALTER ANY SCHEMA at the database level, and that is why your user was able to drop the schema. Removing these two permissions will prevent the role-associated users from creating and droping the schema (unless they have higher level permissions of course).

The required permission to CREATE ALTER and DROP other objects is the CREATE permission for the object type (table\procedure\function\view) combined with ALTER permission on the schema.
You already have these permissions in your script, so all that you have to do is remove the CONTROL permission. For reference, here is a BOL list of DDL statements where You can find the required permission for all object types.

For the lazy, here is your code after removing the unnecessary permission:

CREATE ROLE myrole AUTHORIZATION dbo;
EXEC sp_addrolemember 'myrole', 'myuser';

CREATE SCHEMA myschema AUTHORIZATION dbo;

GRANT ALTER, DELETE, EXECUTE, INSERT, REFERENCES, SELECT,
          UPDATE, VIEW DEFINITION ON SCHEMA::myschema TO myrole;

GRANT CREATE TABLE, CREATE PROCEDURE, CREATE FUNCTION, CREATE VIEW TO myrole;

I can guess you have the AutoClose option for database set to True. This is the default behavior when you create a database with Express Editions.

The mentioned error can occur exactly in this case. Actually the complete error message 615 states: "Could not find database ID %d, name '%.*ls'. The database may be offline. Wait a few minutes and try again." ... So it points that database could be closed during dropping.

So, go to DB properties, switch it to False and try again dropping it or use below script before dropping

ALTER DATABASE [MyDB] SET AUTO_CLOSE OFF 
GO

Many point out that is better to have AutoClose set to False. I found this article explaining a bit more about AutoClose: http://sqlmag.com/blog/worst-practice-allowing-autoclose-sql-server-databases

Small extension of the answer:

-- this works in standard SQL Server Editions, but NOT with Express Editions:
CREATE DATABASE [MyDB]
GO
DROP DATABASE [MyDB]
GO

-- this works in ALL SQL Server Editions
CREATE DATABASE [MyDB]
GO
ALTER DATABASE [MyDB] SET AUTO_CLOSE OFF 
GO
DROP DATABASE [MyDB]
GO